L's PROFILE
Nickname: L 
Aliases: Ryuzaki, Hideki Ryuga, Eraldo Coil, Deneuve
Age: 25 at death
Gender: Male
Species: Human
Date of Birth: October 31, 1979
Date of Death: November 5, 2004
Occupation: Detective
Title: World's Greatest Detective
Blood Type: Unknown
L in Death note
L suspects early in the investigation that Light Yagami is Kira, but can not obtain any concrete evidence. In several situations, Light's seems to be proven innocent by the circumstances, but L remains skeptical. If Light were Kira, the case would be solved, and L has trouble letting go of this simple and complete solution. L can easily present his conclusions publicly at any time, and with his reputation, is taken seriously.
Because of his initial suspicions of Light, L decides to join To-Oh University, the university that Light attends, using the name of Hideki Ryuga for investigative purposes. L attends To-Oh because he feels confidence in his reasoning skills and wishes to test them. He also intends to tell Light that he can "take the offensive." L understands that his act is dangerous and since he understands Light's thought process he feels confident that Light will not kill him at that point.
After L introduces himself as Ryuga, Light does not attempt to kill him since the name is conspicuously fake. Also, he has no way to know at the time whether the person is the real L or a proxy acting under orders. Light also reasons that L would have already introduced himself to Light's father earlier, and thus that if he somehow killed L, he would be easily uncovered as Kira. When Light discovers his "Hideki Ryuga" alias, Light fears that if he writes this name in the Death Note, the face of the pop singer from which the alias was derived would enter Light's head and therefore Light would inadvertently kill the pop idol. This would lead L to the obvious conclusion that Light is Kira.
"Whenever L gives percentages to indicate how much he suspects a person of committing a crime, such as "five percent," in reality he means over 90 percent. Although L gives Light a low percentage indicating how much he suspects Light, in reality L is "pretty certain" that Light is Kira."
In Chapter 58, Light tricks Rem into killing L for his own personal gain, making her believe that she must kill him for the sake of Misa's life. L's last sight before death is Light triumphantly smiling over his defeated rival, and in his last moment L confirms to himself that Light is the first Kira. In episode 25 of the anime, prior to L's death Light and L talk to each other for one last time on the roof of the investigation building; some viewers believed that L, somehow, knew that his death was drawing near.
In chapter 38 of the manga, L suggests that even though he eats only sweet foods, he remains underweight because the brain uses the most calories of any organ in the body. He also says that if he sits normally, his reasoning ability would drop 40%
L in the film series
As there is no time delay between the first and second films, the second film begins soon before L supposedly dies. L retains many of the same characteristics seen in the original series. At the end of the first film, L eats potato chips in front of Light; Light had used a potato chip sack to hide a mini LCD television from security cameras while L filmed his room.
Unlike the manga and anime, in the second film, his original "death", caused by Rem writing his name in her Death Note, does not take effect, as L had written his own name in Misa's Death Note earlier. It is his self-sacrifice that allows himself stay alive for a longer duration (as he supposed Kira would kill him on that day), and allows the investigation team to apprehend Light. After Ryuk kills Light, Ryuk offers L the Death Note. L refuses, and Ryuk, while saying that L is boring, leaves.
Twenty-three days after writing his name in the Death Note, and after burning all the remaining Death Notes and having a conversation with Soichiro, L dies peacefully while eating a chocolate bar, with a picture of Watari lying right beside him. He is portrayed by Ken'ichi Matsuyama, who also voices Gelus (Jealous) in the anime.
L in L: CHANGE THE world
In this film which takes place in the 23 days that L has left before he dies, L decides to solve one last case. He is in charge of a case in stopping a deadly virus from spreading across the world. He meets the daughter of a doctor who was striving to find an antidote and a boy who was the sole survivor of a village destroyed by the virus; after solving the case, he brings the boy to Wammy's Orphanage and names him Near. Because L wrote his own name in the Death Note specifying his manner of death, he is not harmed by the virus, and he uses this to his advantage. The movie also shows that there are other agents with single letter names, presumably all trained by Watari.