In the alternate universe depicted in The Disappearance of Nagato Yuki-chan, Itsuki Koizumi is a normal high school student who attends Kouyouen Academy, possessing none of the esper abilities that define his counterpart from the original series. His role in the narrative is notably smaller and serves almost purely as comic relief, though his defining characteristic becomes his dramatically exaggerated devotion to Haruhi Suzumiya. Unlike the more calculated and mysterious figure of the SOS Brigade, this version of Koizumi is openly and unabashedly infatuated with Haruhi, to the point of unquestioningly following her every whim and command. He treats himself as her willing lackey, seeming to derive genuine satisfaction from supporting her often outlandish schemes and demands. What makes this portrayal particularly poignant is that Koizumi is fully aware his feelings are unrequited. He admits to Kyon that he holds no hope of Haruhi ever returning his romantic interest, yet this knowledge does nothing to temper his enthusiasm or dedication to her. His motivation stems from a foundational event in his personal history: when he transferred to the school and was initially ostracized by classmates, Haruhi was the one who approached him and treated him with kindness, setting him apart from everyone else. This act of acceptance solidified his lasting loyalty and affection.

Despite his hopeless romantic situation, Koizumi maintains his characteristic polite and affable demeanor, often speaking in a formal and friendly manner. He develops a camaraderie with Kyon, who serves as the primary point-of-view character, and the two often find themselves as the only male members of their expanded social circle. Koizumi functions as a supportive friend to Kyon as well, helping to navigate the chaotic energy that Haruhi brings into their lives. In terms of personal abilities, he has no supernatural powers in this universe, being an ordinary human whose notable traits are his pleasant appearance, his friendly personality, and his willingness to go along with whatever adventure Haruhi has planned. He fits the archetype of the Dogged Nice Guy, someone who remains devotedly by the side of the person he loves even with no expectation of reciprocation, finding his role as her devoted assistant to be a rewarding purpose in itself. His presence in the story adds a layer of gentle, uncomplicated affection amidst the more complex romantic tensions between other characters, and his cheerful readiness to serve Haruhi's whims makes him a consistent source of gentle humor.