Itsuki Koizumi, a transfer student recruited into the SOS Brigade for embodying the leader’s ideal of a “mysterious newcomer,” serves as an esper affiliated with the Agency—an organization monitoring phenomena tied to the leader’s subconscious reality-warping powers. The Agency operates under the belief that the world manifests her will, requiring constant stabilization of her emotions to avert disasters. Three years before joining the Brigade, Itsuki developed psychic abilities triggered by “closed spaces”: unstable dimensions spawned during the leader’s emotional turmoil, teeming with destructive “celestials” the Agency must neutralize. While combating these threats, he maintains a cheerful, compliant facade to appease the leader, masking intellectual curiosity and a calculated politeness he privately admits is a stress-management tactic.

Academically gifted, Itsuki enjoys debating paradoxical theories but cloaks serious statements in humor, obscuring his true motives. Despite his composed demeanor, he resents the leader’s exclusive reliance on another member for emotional stability and envies peers involved in time-travel missions. In an altered timeline stripped of supernatural elements, he exists as an ordinary student who openly confesses unrequited feelings for the leader, attends a different school, and parodies loyalty to her for comedic effect—contrasting his primary self’s guarded vulnerability.

Strategically adept, Itsuki engineers scenarios like a fabricated murder mystery to entertain the leader and manipulates school hierarchies by installing a puppet Student Council president to redirect her attention. His powers, limited to closed spaces, include energy-based attacks and supernatural detection. Interactions with Brigade members fluctuate: he shares camaraderie-laced tension with the male protagonist, teasing him about his bond with the leader while quietly resenting exclusion from key decisions. He cautiously allies with time traveler Mikuru Asahina, distrusting her conflicting organizational ties, and monitors alien interface Yuki Nagato’s actions.

A pivotal alternate-timeline confrontation sees Itsuki admitting jealousy over the protagonist’s connection to the leader, unveiling emotional depths beneath his detached persona. Later arcs highlight his awareness of rival factions and pragmatic mediation between the Brigade and external threats, cementing his role as both strategist and reluctant diplomat.