Peter.

Peter Petrelli: "It happened two more times. Look, sometimes I'm flying, sometimes I'm falling - sometimes you're in them!"
Nathan Petrelli: "I don't have time for this now."
Peter Petrelli: "These aren't just dreams, Nathan - I thought they'd go away, but they're not! This morning, when I got out of bed, my foot hovered before it hit the ground -hovered! - for a split second, like I was floating! I'm telling you, I think I can fly!"
Nathan Petrelli: "Tell you what, you think you can fly? Why don't you jump off the Brooklyn Bridge, see what happens."
Peter Petrelli: "I was thinking I ought to start with something a little lower, first, just like learning to walk."
Nathan Petrelli: "You're serious?"
Peter Petrelli: "Oh, I'm serious."
Nathan Petrelli: "You need to snap out of it, Peter - see a doctor, get some drugs, but you do not pull a Roger Clinton on me, man, I'm eight points down in the polls!"
Discoveries.
Peter Petrelli is a nurse living in New York City. He is sensitive and compassionate, wears his heart on his sleeve and is described as a "dreamer". Peter is also defined by his complex relationship with his older brother, Nathan, to whom he feels very close, even claiming he has a sort of metaphysical connection that allowed him to know immediately when Nathan suffered a serious accident in the past. Differences between Peter and Nathan, an ambitious politician who is so focused on his congressional campaign that he shows no compassion when their mother is arrested for shoplifting, have caused tensions between the brothers.
These tensions are exacerbated when Peter starts having extraordinarily vivid dreams of flying. He tries to confide in Nathan, but his brother tells him that the dreams are a sign of psychological distress. Eventually Peter summons Nathan to witness as he jumps off a building to prove he can fly. However, it's Nathan who flies, catching Peter as he falls. Peter blacks out after the brothers lose their grip on each other, waking up in the hospital with muddled recollections of what happened. Nathan claims neither of them flew and that Peter simply jumped, nearly killing himself. Between this and his mother's revelation that Peter's late father had been mentally unstable as well as suicidal, Peter is led to believe he's going crazy.
Drawing a crude stick figure image with one figure levitating a few feet above the ground, Peter gets an idea. Attempting to discover the truth, Peter stands on the edge of another rooftop later that evening. Nathan arrives, and Peter threatens to jump if Nathan doesn't tell him what really happened. Nathan admits that they both flew: Nathan first caught Peter, lost his grip, and then Peter flew some way down to the ground before falling. Peter at first doesn't believe it, thinking Nathan is only telling him what he wants to hear, but then Nathan points out to Peter that Peter is levitating as they argue. However, Peter's later attempts to fly, when he is away from his brother's presence, are initially unsuccessful.
Peter decides to quit his job as a nurse, saying that he no longer feels right "trying to save the world one person at a time" but is meant for something more. As he is leaving, he confides to Simone Deveaux, the daughter of the dying man Peter has cared for, that he has loved her ever since he first saw her. The two of them spend a night together, even though Simone doesn't know how she truly feels about Peter, having just broken up with Isaac Mendez, a tortured painter addicted to heroin.
The first hints of Peter’s greater destiny come when he is contacted by a future version of Hiro Nakamura, a man with the power to manipulate time and space. This visit occurs when Peter is riding the subway, and all the other passengers are frozen in time. Hiro has come to give Peter a message: "Save the cheerleader, save the world." He urges Peter to be "the one we need", saying this was what Peter always wanted to hear, while also noting that Peter looked different "without [the] scar." He finally tells Peter to go and visit Isaac.
Save the cheerleader, save the world.
Peter visits Isaac and convinces him to see past their differences. He learns that Isaac has predicted a nuclear explosion will happen in New York soon, and they both become convinced that saving this unknown cheerleader is somehow connected to averting the catastrophe. While in Isaac's company, Peter also confirms that his superhuman ability involves copying other superbeings' abilities, when he manages to mimic the painter's powers, finishing one painting that apparently shows the cheerleader killed by a figure that resembles Sylar.
Peter has a dream about Simone's ailing father. In the dream, he tells the sick man that he can fly. Simone later comes to his apartment and tells him that her father has died. Before he died, her father told her that he had dreamed of flying with Peter and is worried about the state of the world. However, Peter told him that there are people who care and who will save the world from destruction. Simone then tells Peter that she has sold the painting by Isaac to a man named Linderman in Las Vegas. Peter believes the painting will help him to save the world.
Peter goes to Nathan's house, hoping that his brother can help convince Linderman to give up the painting. When he gets there, he finds Nathan, their mother, and Nathan's family attending a brunch with a newspaper reporter. Peter joins them and is surprised when the reporter implies that Nathan may have cheated on his wife, Heidi, with a blonde in Las Vegas. Peter then claims Nathan was arranging for a private clinic to treat Peter's mental illness. That night, Nathan talks with Peter about what happened in Vegas. Peter is unhappy Nathan cheated on Heidi, but Nathan tries to explain himself. Before he leaves, Nathan says he was temporarily kidnapped, and warns Peter that people will want to lock them up if they realize the two of them have powers. Peter is skeptical of Nathan's story.
Nathan destroys the missing painting created by Isaac Mendez, which depicts a dead Peter. He intends to stop Peter from going to the homecoming game and getting killed. However, Simone shows Peter a photograph of the painting and has taken the liberty of tracking the school down for him. Peter goes to the high school in Odessa, Texas and meets Claire Bennet. Sylar soon shows up to kill Claire, and Peter tries to hold him off while Claire escapes. During their battle, Peter tackles Sylar and the pair fall off the roof of Union Wells High. Sylar survives the fall, but it seems as if Peter hasn't. However, when Claire shows up, Peter mimics her healing powers. As he begins resetting his bones, he asks Claire to call the police. Claire's father has already taken the liberty of doing so, so they show up while Peter is still covered in his own blood and arrest him.
It is later revealed why Peter believes he has a metaphysical connection with his brother when, during a dream, he sees Nathan and Heidi get into the car accident that paralyzes Nathan's wife from the waist down (a scene which also shows Nathan using his ability to fly apparently without meaning to do so). Peter proves there is some hint of a brotherly connection to Nathan when he asks the latter about "the other car" involved in the accident, a detail no one else knew except for Nathan, Heidi, and any persons riding in the other car.
Peter is in jail when he has a vision in which he is visited by his brother, who apparently metamorphoses into Sylar, causing Peter to wake up violently. Later, Peter finds himself using the telepathic power of LA cop Matt Parkman during an interview regarding Sylar's attack on Union Wells High. He tells them Claire Bennet was the real target, and after they interview her she visits Peter, believing he shares her powers. On the contrary, Peter's health is deteriorating for unknown reasons. Claire leaves Peter's cell, telling him that he is her hero; a short time later, Nathan gets Peter out of jail. On their way out of the building, Peter passes out and has a vision in which he sees the other main characters in a city street just before beginning to ignite. As his brother walks up to him, he begins to yell out and explode. After recovering from the vision, Peter tries to explain it to his brother, but stops breathing and falls unconscious.
Waking up.
Soon, Peter becomes comatose. His family watch worriedly by his bedside, and two weeks later he has only gotten worse. As Nathan fears the worst, Peter continues to have the same vision, with some variation. Eventually, a new face appears in his dreams. This man, a tall Englishman in need of a shave, is shown laughing as the events unfold. Another new scene indicates that Peter may be mimicking Ted Sprague's radioactive powers. Eventually, Peter wakes up, and decides that his next destination is "the desert". Out at night, he uses a cell phone and makes travel plans, looking for a place where nuclear testing has been done. Las Vegas is recommended. As he's put on hold, he notices the Englishman from his dreams stealing from a purse. Peter confronts the man, who is shocked that someone can see him. Soon, the man accosts Peter, and tells Peter to leave him alone. Instead, Peter tracks the man, who jokingly called himself "Claude Rains", and asks "Claude" to teach him. "Claude" declines, and tells Peter to stop following him. Peter returns to his apartment to pack his things. Mohinder and Nathan come in before Peter can leave. Mohinder believes that if he is allowed to test Peter he can help him. Peter says he doesn't have time to be Mohinder's guinea pig. Nathan is insistent that Peter not go running off again. To get away, Peter pretends to reconsider, then runs out the door. Seeing an open window, Mohinder and Nathan run down the stairs to catch Peter. However, this was further misdirection, as Peter is actually hiding invisibly with Claude. Claude agrees to train him, but tells Peter that it won't be easy.
Training.
The lessons commence in "Distractions". Claude tells Peter to separate himself from his loved ones. Peter fiercely disagrees, but Claude says that caring about people—particularly Nathan—is holding him back. To instill this point, Claude takes Peter to invisibly spy on Simone. When they find her, she is in a romantically ambiguous embrace with Isaac, after telling him that she believes in him. Later that evening, Claude reminds Peter that he must learn to harness the powers he's previously mimicked intentionally, and without their inherent users present. Then, Claude pushes Peter off a tall building, and tells him to fly. Peter falls onto a taxicab and suffers seemingly fatal injuries, but survives by remembering Claire Bennet and mimicking her powers. Peter is both angry and happy, as he believes this new realization proves Claude wrong. Peter realizes that he does not need to push people out of his mind; he just needs to remember how those people made him feel to regain use of their abilities. However, he starts to lose control when he begins to think about Isaac and others, leading Claude to knock him out. It is also revealed that Nathan Petrelli is the real father of Claire Bennet, thus making Peter her uncle.
Peter is shown to have more control over the abilities he has absorbed. During a sparring session with Claude, Peter uses telekinesis to shield himself from a blow, which puzzles Claude since none of Peter's acquaintances has that power. Peter surmises that he must have picked it up from the man who attacked him in Texas (Sylar, although Peter isn't yet aware of his identity). Later in the episode, when Claude and Peter are assaulted by Mr. Bennet and the Haitian, due to a betrayal by Isaac, Peter uses his abilities to stop the projectiles in midflight. As the projectiles fall to the floor, Peter throws Claude over the rooftop and jumps off himself, only to fly away, carrying Claude in his arms, moments later. After their escape, Claude blames Peter for bringing Bennet and Haitian after him (revealing that he has remained invisible in order to elude them) and leaves Peter on his own.
Peter then confronts Isaac about his betrayal but Isaac is adamant that what he was doing would save the world. Peter concludes that Isaac is jealous of Simone and him and wants to ruin their relationship. When Isaac tries to shoot Peter with the gun he becomes invisible and starts hurling objects around the room. Isaac becomes so spooked by Peter that he accidentally shoots Simone twice in the chest when she suddenly arrives to see him. Peter cradles Simone in his arms as he and Isaac watch her die.
Peter goes to see his brother, who makes an anonymous call to the police about Simone and suggests that Peter go to see Mohinder Suresh. When Peter arrives, he is attacked by Sylar, who begins to slice open Peter's head. Before Sylar can kill Peter, however, Peter begins to regenerate and Sylar pauses. Peter then uses his telekinesis to throw Sylar across the room and immediately becomes invisible. Sylar uses his telekinesis to spray shards of glass around the room, and one is embedded into the back of Peter's head, killing him and making him visible. Mohinder renders Sylar unconcious and escapes with Peter's dead body, taking it to Nathan's home. Everyone believes him to be dead, but Claire finds and removes the shard of glass in the back of Peter's head, allowing him to recover.
How to Stop an Exploding Man.
In the episode "The Hard Part", Peter begins to take a more active role in the events that are unfolding before him. He is trying to convince Claire to stay because he believes that she is the key to keeping him from exploding. When he and Claire go to see Nathan they both see that he is talking to Thompson, Mr. Bennet's former boss. Claire then proceeds to tell Peter that Thompson works for Linderman. They now suspect that Nathan is in league with Mr. Linderman. After this discovery they decide to wait for Ted who is currently in New York with Mr. Bennet and Matt Parkman trying to get rid of Linderman's new tracking device at the place Peter drew with Issac's power. The moment Peter comes in range of Ted, Peter's hands begin to glow. Peter manages to reign the newly-absorbed power in, then decides to leave the city with Ted and Claire while Mr. Bennet and Matt destroy the tracking system.
While attempting to leave the city, Sylar interferes and reports Ted's whereabouts to the FBI. When they come to arrest Ted, Peter escapes with Claire using his invisibility to cloak them. Their next attempt to escape is also inadvertently thwarted by Sylar, who attacks the prison transport holding Ted on the main road out of town. Realizing that Sylar now has Ted's power, Peter assumes Sylar will become the bomb instead of him.
In the season finale, Peter eventually confronts Sylar face to face in an attempt to stop him. Matt Parkman and Niki Sanders intervene as well, and Hiro is the one to finish Sylar off by impaling him with a sword. However, instead of Sylar becoming the bomb as he had thought, Peter's own lack of control leads to him becoming the bomb instead. Although he does become the bomb, Peter doesn't destroy the city. Nathan, having been convinced to save his brother by Claire, flies Peter into the stratosphere, allowing him to detonate harmlessly above New York. Immediately after the explosion, Peter flies up to and catches a falling and radiation-burned Nathan.
Amnesia.
After taking his brother to a hospital, Peter is apprehended by Bob and Elle and taken to a Company facility. Peter is told that the Company is developing a cure for his "disease," and that it would be best for him to stay in the facility until then. Peter, not willing to endanger any more lives after the near-miss at Kirby Plaza, readily agrees. Part of the treatment includes pills to suppress his powers, which Elle calls "Haitian pills."
Aside from dealing with an obviously smitten Elle, Peter leads an unremarkable life until he comes into contact with the inmate in the adjacent cell, Adam Monroe (who had been known as Takezo Kensei to Hiro Nakamura in feudal Japan). The four-hundred-year-old Adam insists that the Company has no interest in helping anyone. After Bob refuses to allow Peter to see his family, Peter comes around to Adam's point of view and covertly stops taking his medication. After five days, he is able to use the phasing ability he has acquired to pass through the wall between his and Adam's cell. After the two finally meet face-to-face, they use the phasing ability to escape from the facility.
Fulfilling a promise he made to Peter, Adam goes with him to see his brother Nathan, who is still recovering from Peter's meltdown and explosion in a Manhattan hospital. Mindful of his regenerative powers, Adam injects his blood into Nathan's IV, and the elder Petrelli's wounds begin to heal almost instantly. Knowing that Nathan's bedside would be the most logical place to find the pair, Adam urges Peter to flee quickly. They make haste but are confronted by Elle and the Haitian. Adam tells Peter to meet him in a warehouse in Montreal, and the two split up. Peter is pursued by the Haitian, who corners him in an empty container. Peter resists but is told by the Haitian that he deserves a better life than one spent imprisoned by the Company. The Haitian says that Peter's mother helped him in his time of need, so he is returning the favor. Peter does not understand what sort of alternative the Haitian has in mind; the Haitian puts his necklace around Peter's neck and erases his memory.
While Peter's family and friends mourn his death, an amnesic Peter is found half-naked and handcuffed inside a cargo container at a port in Cork, Ireland, by a group of thugs led by Ricky McKenna. The thugs accuse Peter of stealing the cargo container's alleged contents, which they had intended to take. After he and his henchmen beat him, Ricky is finally convinced that Peter is suffering from amnesia. He tells Peter his name and reveals that he has a box containing items (which were found on Peter's person) that will provide clues to Peter's former identity (credit cards, driver's license, etc.). However, Ricky will only hand over the box if Peter helps him and his gang with a job. Upon accepting and completing the job - by helping the gang rob a gambling house - Peter earns the box but is initially unwilling to open it for fear that he might not like who he was before. Caitlin, Ricky's sister, encourages him to do so, stating that she believes him to be a good person. The two quickly form a romantic relationship.
In the episode "Fight or Flight," Peter experiences a sudden resurgence of Isaac's precognition power, painting a picture of a street intersection in Montreal, specifically the intersection of Boul. Saint-Laurent and Boul. Saint-Jacques (spelled Boul. Saint-Jaque in the painting), with a building that appears to be La Cathedrale de Notre-Dame in the background. Peter is unaware of it, but this is the warehouse that Adam had told him to meet him at two weeks prior (in "Four Months Ago"). In "The Line," Peter and Caitlin travel there and find a note from Adam warning Peter about the Company. Peter accidentally jumps ahead in time with Caitlin to a desolate Times Square in New York City. A flyer on the ground, dated June 14, 2008, explains that Homeland Security had ordered an evacuation of the entire city.
Shanti Virus.
The virusIn "Out of Time", Peter and Caitlin are captured and separated by a bio-hazard team in 2008. After learning that he has already been killed by the virus, he meets his mother. His mother tells him that he is the one who can stop this future and save the world. When Peter sees Caitlin, they meet and hold hands though the fence barricade separates them. Before he can help her, he unwillingly returns to the present, leaving Caitlin trapped in the future. Repeated attempts by Peter to return to the future fail. In his attempt, Peter is startled by someone entering the Montreal safehouse and uses his acquired electricity power to attack the intruder. The person blocks the attack with his hand, which regenerates, and reveals himself to be Adam. He curiously asks why Peter attacked him, quickly discovering Peter's amnesia. He helps Peter to restore his memories by encouraging him to use his acquired healing abilities to repair the damage to his mind.