Rumina Ayukawa is a 12-year-old Japanese grade-schooler with light brown hair. Bullied by friends into challenging Baki Hanma, he experiences visible distress, crying when intimidated at Baki's school. Despite his timid nature, Rumina gathers the courage to find and confront Baki directly after learning his location.

During the confrontation, Rumina threatens Baki with a knife, showing aggression fueled by external pressure. Baki knocks Rumina flying across a body of water with a single blow, then compliments him for withstanding the hit, acknowledging his resilience. This recognition from a skilled fighter marks a significant moment for Rumina. Baki subsequently takes Rumina to his residence and demonstrates shadow boxing techniques, giving the younger boy an impromptu lesson in combat visualization.

Later, Rumina observes Baki describing the strength of his father, Yujiro Hanma, noting undertones of admiration in Baki's account despite their adversarial relationship, revealing Rumina's perceptiveness about complex family dynamics. Rumina's documented story is confined to this encounter and its immediate aftermath.