Koizumi features in the anthology's second episode, "Transistor Smartphone," within a vignette centered on her brief but turbulent high school friendship with Kumeno. The narrative traces their fleeting connection, which fails to solidify into a lasting bond, examining themes of missed opportunities and transient relationships against the series' broader motif of departures and journeys.

Portrayed as a high school student, her interactions with Kumeno are marked by unresolved tension and a lack of closure. This dynamic reflects the episode's exploration of abruptly terminated relationships and underscores the emotional complexity of fading youthful connections.

Koizumi's role remains confined to this episode, with no additional background, development, or personal details provided. She functions solely as a narrative device illustrating impermanent human connections.