Devils

In the Devils' Line universe, devils are a subset of human that share a lot of similarities to vampires, but are more similar to humans. They live normal lives identical to those of humans except that if they see blood, they transform into monsters and lose control, causing them to attack and suck the blood of those around them.

Appearances
Normally, the devils' appearances do not differ from humans. They are biologically very similar to humans and have characteristics of what every human has. However, it is notable that non-blood-drinking devils have heavy dark bags under their eyes, which is a way to identify them. Devils who drink blood does not have eye bags, such as Johannes Kleeman, and some devils also choose to use makeup to hide their eyebags, such as Juliana Lloyd. When devils transform, their face and body features change drastically. Their eyes will turn red with veins sometimes appearing around the eyes. Their teeth will sharpen into fangs and their nails into claws.

Johannes Kleiman is the only (half) devil who only has one transformed eye.

Personality
Just as they are almost biologically similar to humans, devils also possess human emotions and are capable of showing care for others. As shown with Akimura who genuinely cared for Tsukasa, he expressly said that he wants to keep her safe from harm. Akimura also confessed that he would feel bad if Tsukasa gets a boyfriend. One time, following his string of murders and rape of women which he did to satiate his hunger and lust so that he would not direct them towards Tsukasa, he wondered which is the right thing to do: 1) continue killing other women in order to prevent him from harming Tsukasa and thus keep her safe, 2) kill Tsukasa so as to prevent him from killing more innocent women, or 3) get himself killed so that no one else will have to die. This shows that devils possess human conscience, albeit overshadowed by their natural blood lust, and are capable of normal human emotions such as love, compassion, regret, and jealousy.

The most notable distinguishing factor between regular humans and devils is the presence of blood lust in devils. Although there are so-called non-drinkers, devils who suppressed their hunger by completely abstaining from consuming human blood, even they are prone to sudden difficult-to-control blood lust upon exposure to fresh blood from an open would. This is seen when Anzai lost his cool and reached out to lick the blood from a scratch on Tsukasa's face.

Also, this blood lust is very closely associated with sex drive. When a vampire (devil) drinks the blood of a human, he is overtaken by a strong sexual desire which compels him to sexually assault his victim. For instance, Akimura was mentioned to have raped his three female victims and his semen was found on their corpses. Anzai too, after licking the open wound on Tsukasa's face, was immediately moved to french kiss her, albeit against her will.