Ok, so here's my attempt at answering this question: Walter may or may not be some kind of vampire in Hellsing. He is changed by the Major into SOMETHING...but we never see him drink blood, and he seems to be a different creature, maybe more along the lines of what Schrodingers cat is, who is everywhere and nowhere. We know Walter regains his youth and his strength, and in general surpasses his previous fighting ability. So, in brief, who knows what Walter is, but we know he has been changed by some means into a traitor and a deadly fighting force.