L is quite secretive and only communicates with the world through his assistant Watari. He never shows his face to the world, instead representing himself with a capital L in Old English Five or cloister black. After meeting the Kira investigation team, he requests that the task force refer to him as Ryuzaki for discretion. The Kira investigation team never learns his true name. Ohba said that L is the smartest character in the manga because "the plot requires it." Ohba also said that he personally views L as "slightly evil."

L's disheveled and languid appearance masks his great powers of deduction and insight. He tends to second-guess everything he is presented with, and is extremely meticulous and analytical. He has several quirks such as preferring to crouch rather than sit, eating only sweet foods like desserts and fruits, and holding items very delicately. L stayed in England for five years, and during that time he was the champion in tennis for the England Junior Cup. L also secretly assumes the identities of the world's second and third best detectives, (Eraldo Coil and Deneuve respectively).

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. Light also fears that if he writes Hideki Ryuga into the Death Note, that the face of the pop singer from which the alias was derived would enter Light's head, and Hideki Ryuga would die by accident, which would lead L to the obvious conclusion that Light is Kira.

L also uses the aliases Hideki Ryuga (流河 旱樹, Ryūga Hideki), Ryuzaki (竜崎, Ryūzaki), Eraldo Coil (エラルド＝コイル, Erarudo-Koiru), and Deneuve (ドヌーヴ, Donūvu), the latter two for which he has developed reputations as the second- and third-best detectives in the world.

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. Ohba said about L, "The truth is, he's a liar. [laughs]"

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.

Death Note: How To Read 13 revealed many facts about L. His true birth name is L Lawliet. His date of birth is Halloween of 1979 (1982 in the anime), making him 25 when he dies on November 5, 2004. His height and weight are estimated to be 179 centimeters and 50 kilograms (5 feet and 10 inches and 110 pounds). His blood type is "unknown." It is implied that L was a former resident of Wammy's House, one of Watari's orphanages, as well as a safe haven and educational facility specifically for intellectually-gifted children. Therefore, similar to Near and Mello, L's abilities were discovered while living there, and he is most likely an orphan.

When asked about L's nationality, Ohba responded saying "I think of him as a quarter Japanese, a quarter English, a quarter Russian and... maybe a quarter French or Italian? Something like that."

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%.

Ohba said that his favorite human character in the series was L as Ohba believed that L was the "strongest" character in the series "besides Light." Obata chose L as his favorite due to "appearance, personality, everything." Obata said that he could never have created L and that he enjoyed drawing L. Obata added that because of this, L is not "real" to him and therefore he likes that aspect of L. In response to the question "Who was the most fun to draw?" both Ohba and Obata answered "L."

Many fans of the series believe that L has Asperger's Syndrome, due to the fact he acts like a child at points (not wearing socks, eating lots of sweets, and seeing things somewhat like a game). This could be the author's intention, but that is for the readers to decide.