Prior to joining North High, Koizumi was just an ordinary student, until gaining his esper powers shortly after Haruhi Suzumiya unknowingly altered reality three years before the start of the series. Like many other espers, he was recruited by the Agency, a secret organization dedicated to observing and containing supernatural disturbances caused by Haruhi’s subconscious power.

Recognizing Haruhi’s potential to reshape reality at will, the Agency assigned Koizumi to integrate himself into the SOS Brigade to keep a close watch on her actions.

Unlike Yuki Nagato and Mikuru Asahina, who also monitor Haruhi from different organizations, Koizumi takes a more philosophical stance, often suggesting that Haruhi’s power could be a blessing rather than a curse.

Koizumi is a tall, handsome young man with light brown hair and deep brown eyes, giving him an attractive and approachable appearance. His well-mannered posture and calm expressions make him seem like the perfect gentleman, though his ever-present smile often makes him seem suspiciously composed.

He wears the standard North High uniform, consisting of a teal blazer, white dress shirt, and red tie, which he always keeps neatly arranged, reinforcing his polished and refined image.

Itsuki Koizumi maintains a polite, charming, and unflappable demeanor, often engaging in lengthy explanations and philosophical debates, frequently using expressive hand gestures. However, as revealed in Love at First Sight, this composed exterior is largely an act beneath it, Koizumi experiences stress and frustration, though he rarely lets it show.

Highly intelligent and articulate, Koizumi enjoys arguing contradictory theories purely for rhetorical amusement. He has a habit of following up serious or cryptic statements by telling Kyon that he was “just kidding,” leaving it ambiguous whether he was joking or attempting to downplay something important.

Academically, he excels in all subjects, further adding to his reputation as a model student. Despite his intelligence, he is notably bad at board games and card games, though in the anime adaptation of Remote Island Syndrome Part 1, he unexpectedly defeats Kyon in a game of “Old Maid.”

At North High, Koizumi is very popular with the female students, thanks to both his good looks and his gentlemanly behavior, though he never appears to reciprocate romantic interest in any of them.

Itsuki Koizumi is an esper and a member of The Agency, a secret organization dedicated to monitoring and managing Haruhi Suzumiya’s subconscious reality-altering abilities. His primary role is to counteract the effects of Closed Space, special areas that appear when Haruhi experiences emotional instability. Within these spaces, giant beings known as Celestials manifest and threaten to reshape or destroy reality. To prevent this, Koizumi and his fellow espers enter Closed Space to suppress these disturbances.

As an esper, Koizumi possesses a range of supernatural abilities. He can fly and move at the speed of thought, transforming into an energy-based form that enhances his agility and combat effectiveness. In battle, he generates powerful energy spheres, which he uses as projectiles or explosive blasts to combat Celestials. While his full esper abilities only function within Closed Space, he has demonstrated the ability to create small but concentrated energy spheres outside of it. Additionally, Koizumi can sense supernatural activity, allowing him to detect disturbances tied to Haruhi’s emotions even in the normal world.

Despite his immense power in Closed Space, Koizumi and The Agency cannot directly control Haruhi’s abilities. Their main objective is to suppress the damage caused by her unconscious reality-warping rather than manipulate or change it.

Haruhi Suzumiya
Itsuki’s attitude toward Haruhi remains ambiguous. He consistently agrees with everything she says, ensuring that she remains happy and entertained, as this helps prevent the formation of Closed Space. However, The Disappearance of Haruhi Suzumiya reveals that the Alternate Itsuki harbors romantic feelings for her. Given that Alternate Itsuki’s explanation—finding Haruhi “charming”—echoes a comment made by the real Itsuki in The Melancholy, it is implied that the real Itsuki might also have similar, albeit unspoken, feelings for Haruhi. Furthermore, during The Disappearance, when Haruhi remains by Kyon’s side in the hospital, the real Itsuki repeats the Alternate Itsuki’s line, “Makes me jealous…”, hinting at possible underlying emotions.

Kyon
Itsuki considers Kyon a trustworthy confidant, often providing him with crucial information regarding Haruhi’s powers and supernatural events. However, he frequently invades Kyon’s personal space and enjoys teasing him, much to Kyon’s annoyance. Despite this, Itsuki appears somewhat disappointed by Kyon’s dismissive attitude toward the extraordinary, as well as his general indifference toward him. He has even complained that Kyon’s behavior resulted in mockery at “the office.”

Socially, despite Kyon’s irritation and occasional distrust, they get along well as the only two males in the SOS Brigade. Itsuki often takes an interest in Kyon’s dynamic with Haruhi, frequently suggesting, both jokingly and seriously, that there is romantic potential between them. In The Surprise of Haruhi Suzumiya, he directly encourages Kyon to ask Haruhi out on a date, stating that he hopes a relationship between them will develop. He also firmly believes that Kyon is the only person Haruhi truly listens to and insists that Kyon’s role is to “stabilize” Haruhi’s emotions, thereby preventing the creation of Closed Space.

Mikuru Asahina
Itsuki is highly skeptical of Mikuru Asahina, particularly the Older Mikuru. He believes she cannot be fully trusted, though he is willing to cooperate with her when necessary. He also expresses doubt about the younger Mikuru, suggesting that her helpless and shy demeanor is merely an act designed to manipulate Kyon. However, in Love at First Sight, he later backtracks on this claim, leaving Kyon unsure whether Itsuki was serious or merely provoking him for fun.

Additionally, some hostility exists between them, as the factions they represent The Agency and the time travelers hold contradictory theories about Haruhi’s nature and the extent of her power.

Yuki Nagato
Itsuki keeps a close watch on Yuki’s actions, although not as closely as Kyon does. Kyon has noted that Itsuki carefully follows Yuki’s every move, implying a level of interest or respect toward her abilities.

Itsuki has admitted that Yuki rarely responds to his inquiries, often remaining silent when he tries to communicate with her. However, she follows his instructions when necessary, as seen in The Sigh of Haruhi Suzumiya, when he asks her to destroy a fence, or in Snow Mountain Syndrome, when she keeps track of time per his request. Occasionally, she provides him with factual answers, especially regarding supernatural occurrences, though her responses tend to be much more frequent and detailed when directed toward Kyon.