Jack is a member of the Phantom Thieves, an organization that emerged following the unexplained vanishing of Lost. He employs explosives as his primary weapons during operations, positioning him against protagonists Yuki and Takuya as they travel toward the disappeared city.

Jack vanished under conditions tied to the catastrophe that annihilated Lost, an event that precipitated his recruitment into the Phantom Thieves. This group, founded by Tsubasa and including Yoshiaki, Rui, Sumire, and Kana, pursues objectives directly opposing those who seek Lost's truths.

As an operative, Jack consistently utilizes bomb-based tactics in the Phantom Thieves' missions. His actions heighten obstacles for Yuki and Takuya through orchestrated confrontations. No narrative evolution alters his motives, allegiances, or personal trajectory.

The circumstances of Jack's disappearance define his only known background catalyst. His relationships extend solely to organizational affiliations within the Phantom Thieves. Details about his life prior to joining, personal stakes in the group’s activities, or ultimate resolution to his involvement remain unexplored.