Kaidou Shima, a supporting character with black hair, brown eyes, and a mature demeanor, frequently wears glasses and blends casual and formal attire. As the younger yet taller twin brother of Aki Kaidou, he balances Aki’s brashness with responsibility, composure, and quiet observation. Among four siblings in the Kaidou household, Shima shares a father with Haru, their half-brother, born from their father’s prior marriage. Following their parents’ fatal car accident, Shima and Aki relocated to Haru’s home after high school. Though Shima detected inconsistencies in the family registry, he pragmatically accepted Ren’s adoption, prioritizing harmony over confrontation.

In childhood, Shima and Aki resented Haru, the accident’s sole survivor, blaming him for their loss. This bitterness waned after Ren’s arrival, with Shima gradually embracing a mentor role toward Ren. He advises Ren on etiquette instilled by Haru and reflects on their shared upbringing, later finding fulfillment in guiding a younger brother. His pragmatic adaptability evolves into genuine familial investment, subtly reinforcing bonds.

Shima attends college with Aki, balancing studies with part-time work at the family café, White Fang. He tutors middle schooler Ai Natsukawa, whose fleeting crush underscores his approachable yet reserved nature. Within the family, Shima mediates tensions between Aki and Ren, his quiet support evident in moments like discreetly checking on Ren’s overnight outings. Though emotionally restrained, his actions consistently prioritize his siblings’ well-being.

Across narratives, Shima’s steady presence anchors the family, countering Aki’s impulsivity. His acceptance of Ren—despite non-biological ties—highlights themes of chosen kinship. While his personal ambitions remain unexplored, his reliability remains pivotal to the household’s unity, embodying a quiet dedication that sustains their collective resilience.