Demon

Demon (悪魔, Akuma) is a collective term describing several creatures believed to be opposed both to and by God.

Information
In Hellsing, a "Demon" is a word that's commonly used to describe supernatural creatures like vampires for example, as turning into one means that you had completely abandoned and betrayed God, along with humanity as well. The Vatican, especially, has immense hatred for those who defies God; which makes these kinds their primary target. Possessed of preternatural strengths and weaknesses, as well as monstrous instincts and an inborn predation towards humanity, demons are common antagonists in the Hellsing universe.

Connection with Vampires
In the vampiric lore and various medias, the first step to becoming a vampire and achieve "immortality" - is to sell your soul to the Devil itself, in other words; abandon the lord, and to an extent; give up everything to the devil, and then finally, drink or lick blood; that way, you can finally become something inhumane. In Hellsing, Dracula, or Alucard, before he became a vampire, was sent to be executed by the Turks by decapitation, however, before they could do so, Dracula had licked a blood that was on the floor and had completely abandoned God, as he felt betrayed because all he had done for him, only led to this moment; so, with no other choice, Dracula licked the blood in the floor and was decapitated, then when the sun goes down and the moon rises, he wakes up and then, brutally murdered the Turks present. The unimaginable sin in which, they made for betraying god - inevitably, the deepest pits of Hell will surely have some space for these creatures that hunts and feeds on humans, innocent or not and for the most part, does it for their satisfaction and desires only.

To put it simply, these vampires had given up everything, even their soul, to the devil; therefore betraying everything - their family; humanity and finally, God. And that what makes them a "Demon", an inhuman species that, for the most part; a pawn for the devil and a threat to humanity. They're no more than selfish and manipulative scum who does everything but good.

Vampires
Vampires are the main characters and antagonists of the Hellsing series. They are supernaturally strong and fast beings that hunt and feed on humans, often controlling a larger army of ghouls. Alucard is the most prominent vampire and a true vampire, followed by Seras, the Draculina. They are divided into two groups: artificial vampires, which were humans converted into vampires by Millennium, and true vampires, which were humans converted into vampires naturally.

Werekin
Werekin are humans that exhibit animal-like qualities. There are very few Werekin ever seen in the Hellsing universe. Only two were ever known: The Captain and Schrödinger, a Werewolf and a Werecat respectively. Those two, possibly the only two, have thrown their lot in with the Millennium Organization. With both The Captain and Schrödinger dead with Schrödinger inside Alucard, it is possible that the Werekins are extinct.

Ghouls
Ghouls are the red-shirts of monsters. They are undead and totally obedient to their makers, the Vampire who bit them. They are a step above zombies, with the ability to control weapons and work as a functional army. However, they still hunger for human flesh and still have the same viral properties zombies do. Shooting them in the heads or hearts spells death for them.

Zombies
Zombies are, for the most part, similar to ghouls; they're stronger than humans but however, dumber than them and can be considered, braindead. However, unlike ghouls, they don't have the ability to use weapons or work as an army, and instead, they mindlessly go around looking for humans.

Known Demons

 * Alucard (Vampire)
 * Seras Victoria (Vampire)
 * Incognito (Vampire, deceased)
 * The Captain (Werewolf, deceased)
 * Rip Van Winkle (Vampire, deceased)
 * Zorin Blitz (Vampire, deceased)
 * Tubalcain Alhambra (Vampire, deceased)
 * Luke Valentine (Vampire, deceased)
 * Jan Valentine (Vampire, deceased)
 * Schrödinger (Werecat, familiar)
 * Letzte Bataillon (Vampires, destroyed)
 * Helena (Vampire, deceased)
 * Laura (Vampire/Baobhan Sith, deceased)
 * Paul Wilson (Vampire, deceased)
 * Cheddar Priest (Vampire, deceased)
 * Leif and Jessica (Vampires, both deceased)
 * Enrico Stivaletti (Vampire, deceased)
 * Mina Harker (Vampire, deceased)