Jack Mouton, born October 22nd in England, appears as a child with neat black hair and black eyes in his human form. After becoming a vampire, his hair typically sweeps back and his eyes turn red. He stands 155 cm tall, and due to vampiric immortality, his physical form remains fixed, never aging. This contrasts sharply with his fraternal twin brother, Robin Laffite, who aged normally after their separation; had Jack not transformed, he likely would have resembled Robin's current adult appearance.

Jack possesses a devilish nature and a strong inclination for mischief, which he openly displays. He consistently seeks attention and enjoys teasing others. When jealousy strikes, he actively works to eliminate the source or assert his superiority over the person or object of his envy. He harbors a fondness for sweets and games, showing a particular preference for cotton candy from Totti Candy Factory on Takeshita Street.

Professionally, he works with Veuve Elizabeth as both an image model and assistant for her clothing brand "Rougelian." Within LOST EDEN, his relationships are complex; he considers Mist Flaive his guardian yet often finds him irritating. Jack joined LOST EDEN before Mist. He deliberately changed his surname from Laffite to Mouton to evade police attention; "Mouton" is French for "sheep" and also refers to 14th-century French gold coins. Saga Latour did not sire Jack, as Saga exclusively turned Mist into a vampire.

Jack's documented abilities include transforming into mist and the power to summon functional black wings. Narrative elements highlight moments where Jack and Robin occupy nearby locations like Takeshita Street and Togo Shrine simultaneously, though physical separation prevents them from seeing each other.