Line (ライン, sometimes translated as "Rain") is a translucent, orange-yellow colored liquid drug based in Toshima, which heightens the user's instincts to a superhuman extent and gives them unbelievable physical strength. However, Line will eventually cause the user to lose control of themselves and go into a fit of destructive rage before their ultimate death due to incompatibility.
Line is distributed mostly in a 10% concentration, but concentrations of up to 40% exist on the market. However, anything higher than 10% is very difficult to find. The drug is usually taken using a rubber dropper, so that very small doses can be taken.
There is no known cure to Line up until the discovery is made that Akira is the anti-Nicole. A person can be cured by ingesting his blood. However, it is very potent and kills most people who ingest it before it can cure them. The only person known to have been cured using this method is Keisuke, who has an unusually high compatibility for the drug.
It is, however, possible to quit Line, but due to the extreme addictive properties of the drug, doing so would require the help of someone else who can take care of the person trying to quit and cut off their access to Line.
In the Game
Line is used by most Igura participants, as anyone who does not naturally excel at fighting will quickly lose to someone taking it. Most Line users believe that they can defeat Il Re as long as they have Line, despite the fact that Il Re remains undefeated even after being challenged many times.
A 50% concentration of the drug is given to Takeru by Nano, who, after some moral deliberation, decides to take it. In his drugged state, he hunts down Akira, obsessed with showing the champion the humiliation of defeat. Since they were on even fighting grounds previously, with the addition of Line, Takeru easily subdues Akira. While he is tormenting him, he accidentally ingests some of Akira's blood, and starts convulsing, which allows Akira the chance to escape. Before Takeru dies from the blood, he is killed by Keisuke.
Keisuke is also driven to take a high concentration dose of Line after being told off by Akira at the hotel. His feelings of not being good enough manifest themselves as a powerful anger at Akira for "always acting superior," and he massacres everyone at Meal of Duty before he also goes to hunt Akira down. Depending on the route, Keisuke torments and/or kills Akira in several ways. In Keisuke's good end, he accidentally ingests some of Akira's blood, and with Akira's care, he manages to cure himself of Line.
At the end of most routes, it is revealed that Line is actually the blood of the human weapon Nicole Premier, which is diluted to make the drug. It is also revealed that Akira is the anti-Nicole; his blood neutralizes Line. This causes the person who ingests his blood to go into a convulsive fit, and either die if their body can't take it, or have their body cleansed of Line.
In the Anime
Line is mainly used by hoodlums who have entered the tournament in order to defeat Il Re. There are two shown forms of Line, the strong and the weaker version, but Arbitro's words suggest that there as many versions as there are possible ratios between the pure substance and water. The weaker version of Line is greenish-yellow, while the stronger version is crimson red.
It is shown that Akira's blood, when consumed by someone who is on Line, makes them collapse to the floor in tremendous pain and usually leads to their death. Later it is revealed that this is because Akira's blood is "Null Nicole", which means that it is the exact opposite of Nicole, or potentially an antidote to it.
It is later found out that Line is actually the blood of a superhuman, Nicole Premier. The weaker a version is, the more water will be mixed to the blood to reduce both its main and side effects. It's revealed that Motomi and Arbitro were actually low-ranked scientists working on a top secret government "Super Soldier" experiment, with the purpose to create an army of super-enhanced soldiers. After the Third Division (World War III) took place, the experiment was abandoned and the only people left that know about it are Arbitro and Motomi.
In the Togainu no Chi Manga
A major subplot involves Shiki trying to discover the source of a few vials of 50% concentration Line, which had sparked fast spreading rumors among Line users. He later discovers that it was Nano himself who had distributed the vials.