A cheerful red-haired Exorcist of mixed race, who aims to become a Bookman and is working along the Black Order just to be close to the events that must be seen. To help him, he developed an incredible memory: he is able to memorize every single aspect of something he saw, but he commented (in chapter 101) that he could identify "the key" (the key which Tyki Mikk gave) faster if he were to use both his eyes when sieving out the real "key" from the fake. He is 18 years old, and normally uses his left eye while the right eye is covered by an eye-patch (no one knows what his right eye looks like, because he hasn't revealed it yet).
He enjoys giving nicknames to various characters, naming Bookman "Panda" due to the thick black circles around his eyes, a converted Akuma "Chomesuke", and Arystar "Kuro-chan" in the anime and "Kro-chan" or "Krorykins" in the manga. He is the only character other than General Tiedoll to call Kanda by his given name (that name being Yu, also translated as Yū or Yuu), and apparently speaks to him outside of missions, a fact that was shown when he called Allen "beansprout" ( モヤシ , moyashi), saying that if Kanda could call him that, Lavi could as well, as he is the same age as Kanda. He has a habit of teasing Allen, and deems himself as his big brother. A recurring gag in the series is Lavi's fondness for beautiful older women, whenever he sees a woman his type (Example: Eliade, Lulubell, Chomesuke, Anita) he'll yell out "Strike!" with pink hearts in his eye.
It is later revealed that his name, Lavi, is just an alias (his 49th, to be exact). In episode 88 of the anime, it is revealed that Deak or Deke ( ディック , Dikku) was his 48th alias. He had cast aside his old name and background to show his commitment to being a Bookman. It was also revealed that Lavi was once cold and indifferent to others, for a Bookman only lives to view and record history and that humans are nothing more than 'ink' on a page. However, over time, Lavi's personality changed and he began to truly care about others, slowly casting aside his identity as a Bookman as shown when he thought that Allen was killed by Tyki Mikk and tried to avenge Allen by attempting to kill Tyki himself.
Lavi is introduced into the series after the The Rewinding City arc alongside Bookman. He is the one that shows Allen how hard it is for "normal" Exorcists to find the Akuma. Along with Bookman, Lavi joins Allen and Lenalee to find General Cross. When Allen is separated from the group, Lavi is the one that goes back to find him. Lavi then works with Allen when they fight and later befriended a new Exorcist Arystar Krory in the The Vampire From the Old Castle arc.
He later helps Lenalee look for Allen after he's attacked by Tyki. Along with the other Exorcists, he goes to Edo to find General Cross and helps fights off the attacking Akumas. When Lenalee does not return to the ship, he gets deeply worried, so he uses his Ōzuchi Kozuchi(see below) to go look for her. When he did, instead he finds the converted Akuma Chomesuke. He, thinking she was the one that hurt Lenalee he threatened to destroy her. After he finds out the she is on their side, he is forced to apologize to her. Both him and Bookman find out that Lenalee's Innocence saved her life, which has never happened before, meaning hers is an "exception". They start to think that her Innocence might be the "Heart". After arriving in Edo the group is attack by a level 3 Akuma. Using his "Fire Seal" to get rid of two other level 3 Akuma, the now three 3 Akumas then fight each other and the group escape.
After being found by the Earl, Lavi tries to kill Tyki for what he did to Allen, but when the Earl makes the Combined Akuma, he has to help the other Exorcists destroy it, with no success however. After the group is saved by General Tiedoll's group, they are in a sense capture by the Earl's golem Lero along with Kanda. They are found by Tyki in his human form who after turning in to his Noah form gives them a key to use Road's "door". After they use the key they run into Skin Bolic. Kanda then stays behind to let the others go on. They then encounter Jasdero and David who take the key and make copies of it and then make the group find the "real" key while fighting them at the same time. Lavi has the others fight off the two Noahs and using his incredible memory finds the "real" key. Krory stay behind to fight Jasdevi and along with the others Lavi moves on. After they leave the group they run into Tyki and Road. When Tyki reveals that his Tease ate through Allen's heart, Lavi realizes that there are two "exceptions" (the other being Lenalee's). After Allen starts to fight Tyki, Lavi starts his fight with Road who uses her power to make him fight his "past".
During his fight against his past incarnation, his 'heart' (current personality) was supposedly destroyed. This caused his past incarnation, under the control of Road, to emerge in his physical body and savagely attack Allen. Unwilling to hurt Lavi, Allen attacked Road but failed to harm her, as her current body was not her true form. Lavi's current self managed to resurface and suppress his past self. He then engulfed himself in a massive Fire Seal to attack Road in the real world, while simultaneously stabbing her through the heart within her dimension, defeating her.
After the appearance of Cross Marian to their aid, Lavi's status was unknown after falling into a 'hole' made by the Millennium Earl's flashy entrance. He attempted to lengthen his Innocence to save himself, but it shattered when Allen grabbed Lavi's Innocence in an effort to pull him up. Although he was apparently lost in the 'hole' in the Ark, he was shown to have been revived along with the rest of the 'lost' Exorcists on the Ark when the Ark was restored by Allen. He is now back at Black Order HQ waiting for his Innocence to be fixed. When Lulubell and her Akuma attack the HQ, he is unable to assist Allen as he cannot activate his (shattered) hammer. Since he is unable to assist in the battle, he is put into the infirmary with Lenalee, who is also unable to help. Lavi protects Komui and Lenalee with a flag, since his hammer has not yet been repaired. Joined by Kanda and Allen, he fights the level 4 with everything he has, taking a lot of damage in the process, in order to save Komui, Lenalee, and Hevlaska. He was turned into a child just as Kanda because of a potion found during the Head quarter's moving process.past that, lavi is back to his old self and shown coming to the new headquarters for bookman business. He is later shown the be in the room with Cross and Allen observing the told information first hand.
His anti-Akuma weapon is a size-shifting hammer, which is called Tettsui, although he often calls it Ōzuchi Kozuchi ( 大槌小槌 , Ōzuchi Kozuchi lit. "big hammer small hammer"). It can grow ( 満 , man) and extend ( 伸 , shin) at Lavi's command. The weight increases with its size, but since Lavi is the Conformer, this weight change does not affect him. Besides using it for brute strength, Lavi can also use the hammer to summon several "judgments" by hitting elemental seals (written in Kanji) that appear around his head.
Some of the seals have not yet been revealed and appear as question marks interspersed around the seals. This probably means that Lavi has yet to learn some seals from his Innocence. When Bookman berates him for not using the Wood seal to move the clouds, Lavi states that he had forgotten that he could use it to affect nature directly, and as he remembers, the question mark turns into the kanji for the Wood Seal. The Wood Seal can actually be seen throughout the manga whenever Lavi summons his seals (e.g. during the use of the Heaven seal), long before the use of the actual seal itself. The seals for Water and Earth can also be seen, but Lavi has not yet used them