Fifteen-year-old Mirei Colder serves as the primary hacker for STAND, leveraging advanced technical skills to support the team through cyber warfare and systems infiltration. While she abstains from direct combat, her responsibilities encompass monitoring virus activity, analyzing threats, and providing tactical field support. Erika Tinen acts as her protective older sister figure, offering guidance and emotional support. Mirei shares a close dynamic with Jouichirou, who frequently teases her about her visible concern for his safety.

Despite her outward youthfulness, Mirei carries significant emotional trauma from her childhood: the death of her younger brother Leon during an unspecified incident. She internalized deep-seated guilt, believing herself responsible for years. A pivotal moment occurs during a neural implant reformatting procedure where suppressed memories reveal she was not at fault for Leon's death. This realization absolves her of misplaced guilt, marking a turning point. Her relationship with Jouichirou later evolves into romance after she reaches Neo Hong Kong's age of consent, a development primarily explored in unofficial fanworks.

Mirei's technical expertise remains essential to STAND throughout the series. Her interactions with teammates reveal youthful vulnerability alongside intellectual maturity shaped by her past trauma. Her character arc centers on reconciling technical proficiency with emotional burdens, ultimately achieving resolution regarding her brother's death.