Because pretty much every Hellsing character was in some sort of h-doujin made by Hirano.
Alucard = Alucard
Luke & Jan = rapist vampires
Proto-Seras = rape victim
Anderson = sniper priest who likes little virgins
Captain = Kills rapist jews in desert (lol)
Major = pretty much same dude.
Pip = same dude with added h-stuff.