Lavi is a main character and an exorcist affiliated with the European branch of the Black Order, but his true purpose is to succeed his mentor, Bookman, as the next chronicler of world history. He operates under the alias Lavi, his forty-ninth identity, having discarded his original name to signify his commitment to the Bookman Clan's mission of unbiased observation. Despite his position as an exorcist, he and Bookman joined the Order solely to record the war between the Black Order and the Millennium Earl's forces.

Lavi is a young man with bright red hair, green eyes, and a distinctive black eyepatch covering his right eye. He typically wears the Order's standard uniform along with a green bandana and black fingerless gloves. His physical appearance is matched by a consistently cheerful, mischievous, and outgoing demeanor. He frequently jokes, gives playful nicknames to his companions—calling Bookman "Panda," Allen Walker "moyashi" (beansprout), and Kanda Yuu by his first name—and has a habit of yelling "Strike!" whenever he sees an attractive older woman. However, this lighthearted exterior is a learned persona; his Bookman training taught him to suppress emotional involvement and treat people as mere subjects for recording. Beneath the humor lies a deeply reserved and conflicted individual who struggles with the tension between his duty as a neutral observer and the genuine bonds he forms with his fellow exorcists.

His primary motivation is to become the next Bookman, a role that requires him to observe and record all major conflicts without interference. Yet his time at the Black Order causes him to care increasingly for his friends, blurring the line between detachment and compassion. This inner conflict defines his character arc, most notably during his battle against the Noah Road Kamelot, where an incarnation of his cold past self is forced upon him. Lavi ultimately overcomes this threat by reasserting his current, emotionally invested identity, using his own flames to attack Road and nearly sacrificing himself. His development continues as he questions the Bookman ideology and prioritizes protecting his comrades.

Lavi's key relationships include his mentor Bookman, who raised him and constantly reminds him of his duty; Allen Walker, with whom he shares a brotherly bond and deep trust; Kanda Yuu, whom he teases relentlessly but respects as a strong ally; and Lenalee Lee, toward whom he is protective and caring. His interactions with these characters highlight his shift from distant recorder to an active, loyal friend.

In terms of abilities, Lavi wields an Equipment-type Innocence in the form of a hammer he calls Ōdzuchi Kodzuchi or Nyoibo. This hammer can increase in size and weight at his command, and the additional weight does not affect him while it devastates his opponents. More distinctively, he can summon a set of elemental seals—kanji characters that appear in a circle around his head—and strike them with the hammer to unleash powerful attacks. His known seals include Fire Seal (Hi Ban) creating a torrent of flames, Heaven Seal (Ten Ban) releasing a massive lightning strike, Wood Seal (Moku Ban) allowing control over natural elements, and a Combo Seal combining fire and heaven for a stronger attack. There are also water and earth seals among his repertoire, some of which remain unmastered. His Innocence later shatters during the Noah's Ark arc, leaving him unable to fight until it is repaired, and he is subsequently kidnapped by the Noah Clan while Bookman is interrogated for information about the fourteenth Noah. His status remains unresolved in later story points, but his role as a complex, evolving character is firmly established.