Devils

In the Devils' Line universe, Devils (デビル) 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 or taste blood, they transform and lose control, causing them to attack and suck the blood of those around them.

Names
Devils have been called various name:
 * Demons (鬼)
 * Vampires (吸血鬼)
 * The Red Eye Race (紅眼人種)

History
The first officially recorded devil appearance was in 1956, when a vampiric incident murder suspect was arrested in Kanagawa. The following year, someone else was arrested for a similar crime in Saitama. Both of them claimed to have bloodlust, and it was discovered that the smell of blood made them violent. It was initially thought to be infectious, but it was later revealed that it was genetic. Eventually, research into devils secretly moved forward. When the devils who rarely appeared got arrested, researchers took them in and used them to study the physical characteristics and blood relations of devils in order to find out their true nature.

In 1960, there was a request from the government, they wanted the researchers to find out how there could be devils who don't have bloodlust, can't transform and lack the ability to heal. They made a hypothesis that for people who are half or a quarter devil, the transformation becomes insufficient, in other words, they had ReMI (Redeyes Metamorphic Insufficiency Syndrome). Since they had no proof, the decision was made to further research and in 1963, they artificially began developing ReMI, and thus, the Hybrid Project was founded.

Because of how shunned and hated the devils were in society, most devils in the early ages would often hide away to minimize assault and attacks from humans. It was also very hard for humans and devils to reproduce, since the devils always lose control during sex. This is why many devils inbred, which would lead to a weakened genetic diversity for future generations. This is why life expectancy for Devils is considered to be shorter than average humans.

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 Kleeman is the only (hybrid) devil who only has one transformed eye. All other known devils eye's transform.

Although devils are biologically unchanged from humans, all devils are noticeable to have a body temperature that is lower by 10 degrees than usual.

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 bloodlust, 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 bloodlust 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 bloodlust 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 bloodlust is very closely associated with sex drive. When a 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, Shouta Akimura was mentioned to have raped his three female victims and his semen was found on their corpses. Yuuki Anzai too, after licking the open wound on Tsukasa Taira's face, was immediately moved to french kiss her, albeit against her will.

Transformation
When a Devil sees, smells or drinks blood, their brain is consistently activated. At that point, signals generated within the brain trigger the transformation of mouth, eyes, and nails. This procedure can also lead to remember past things since the signal transmissions in the brain is vigorous during a transformation, it affects the region of the brain that controls memory.

All devils transform upon seeing or tasting blood, and even imagining blood can trigger a transformation. This also applies to sexual intercourse and pleasuring stimulation in general as sexual drive is closely associated with bloodlust and vice-versa. However, while some devils lose control by seeing or tasting blood, some devils who are trained, such as Yuuki Anzai, can manage to control themselves to a certain degree if they are not exposed of too much blood. Johannes Kleeman also transforms, but only his left eye (sometimes his other) transform by the sight of blood, but the rest of him is in control. This is why he can have sexual intercourse without transforming, and heal effectively by drinking blood. Since he is used to blood, he can control his bloodlust as he takes a small dosage of blood every day and night. Devils who are not used to blood easily lose control and their senses, making them attack people around them. They also develop a serious addiction if they drink blood, so they are guaranteed to do it again. It is also mentioned that it is very hard to kick off the habit of drinking blood, since once a devil gets the taste of blood, they can never forget it.

Abilities
Devils have a much more sensitive nose than humans, and can recognize smells that normal people cannot. Devils' fundamental physical abilities also surpass that of ordinary humans, even in their untransformed state. When Devils transform, their senses become sharper and the reaction time becomes better. In connection to this, Devils also become more agile and faster which makes them very difficult to defeat since their abilities can be very dangerous. Devils can also make maximum use of their muscle power, almost at 100%. Hybrids also share the characteristics as pure-blood devils, but their bloodlust is noted to be weaker.

However, 99.9% of the time when a Devil sees or taste blood, they lose control and transform, causing emotion change, red eyes development, tooth growth and nail growth, and they will desire to suck blood from humans who are nearby. Therefore, in order to suppress the desire to suck blood, it is necessary for Devils to take measures such as striking a sedative or a tranquilizer. The most notable thing is how quickly Devils heal from a wound, especially if they drink human blood. Devils also don't die unless they are shot in the head.