Itsuki Akiba is a Japanese girl with light brown hair and brown eyes, born October 20. She is Ichitaka Seto's childhood friend, knowing him since grade school. Their friendship ended abruptly when she moved to the United States with her parents four years before the story starts, resulting in no contact. She unexpectedly returns to Japan, re-entering Ichitaka's life suddenly. Itsuki is lively, blunt, and provocative, frequently causing Ichitaka embarrassment. Despite the long separation, she retains strong romantic feelings for him and vividly remembers his childhood promise to marry her.

During their youth, Itsuki often coached Ichitaka emotionally, especially about girls, indicating a bond deeper than typical friendship. After returning, she actively disrupts Ichitaka's romantic pursuits, particularly his interest in Iori Yoshizuki, by openly competing for his affection. She aspires to become a makeup artist specializing in monsters or special effects for films, a childhood dream. Her early talent is shown by a sculpture of Ichitaka she created years prior.

Approaching the narrative's midpoint across all media, Itsuki departs Japan again to chase her career goals, accepting an opportunity to work with a prominent sculptor in America. After her physical exit, she occasionally surfaces in Ichitaka's thoughts during moments of self-doubt or pity. These appearances are not literal returns but manifestations of Ichitaka's subconscious, using her persona to deliver self-critical or motivational messages to himself.