I am Alucard, "True Undead". I serve my human master Integra. I choose to serve her and help the Hellsing Organization rid England of the impure vampires that are appearing.
I've been around for over 2000 years. There is no other undead like me. I mostly serve Integra because I am looking for a "decent" fight. There is no one that is up to my callibar and it is dreadfully boring.
Alucard is a vampire who is under the servitude to his human master, Sir Integra Wingates Hellsing, who he has served for at least ten years or more. Not much is known about Alucard's past, but in some episodes it implies that he had worked for Hellsing dating back before World War II, where he and Walter Dolneaz (retainer of the Hellsing family) had served together against Nazi Germany.
During his time working under Hellsing, he gains an apprentice named Ceras Victoria, saving her from death during one of his missions to kill a vampire. She struggles with her humanity even after she becomes part of the undead.
Alucard in the show has been portrayed as a regular true vampire, but on numerous occasions he implies that he is more than what he seems to be. The producers show a split screen of Alucard now and a past picture of Alucard which bears a remarkable resemblence to the picture of Dracula from Brom Stoker's Dracula, insinuating that he is Dracula the no-life king.