The Doctor (or "Doc") was a character in Hellsing and a member of the Millennium forces. Doc usually appeared dressed in a perpetually bloodstained white labcoat, an unusual shirt which reveals his lower torso, and glasses with an assortment of adjustable, multi-lensed spectacles.
He was Millennium's top scientist of physiology and technology, the knowledge behind many of their projects. Perhaps unstable, certainly eccentric, but undeniably knowledgeable, it was his research that led to the creation of a Nazi vampire army out of 1,000 volunteers in the 'Letztes Battalion'. Hirano has stated that he created Schrödinger, which goes well with his 'mad scientist' reputation. The two argued almost constantly. He appeared in the manga and OVA series, and as a silhouette to the right of the Major in the promotional video (found on the first DVD of the English release). It is possible that the Doctor created the Major's cyborg body, but it is currently unknown as to whether he is also a cyborg or something else.
Per his nametag shown in Volume 3, his name appears to be Avondale Napyeer. The nametag is difficult to read, however, and no other name has been given thus far. It appears that based on Chapter 93 that The Doctor was going to release "She", the enigmatic, heavily bandaged figure from Hellsing: The Dawn. At the end in chapter 94, he was arguing with Walter about how useless he was, that one day he will overthrow and defeat Alucard and that he's being a failed experiment. Doc tries to press a button from his remote to kill Walter. However, Walter uses his wires and cuts off Doc's arm and leg, making him fall to the ground and ripping the blanket off SHE, revealing that she was Mina Harker, who Alucard, as Dracula, forced her to drink his blood.
Doc had, years after Mina's death, exhumed her body and used it to start his experiments in perfecting the Vampire mass-production project, hoping that, one day, Millennium would be able to defeat Alucard (or as Walter said "trying to imitate him"). The Doc, not long after that revelation, was buried under a large heap of falling debris, killing him instantly.