Sunmi is the granddaughter of Daishisai, a figure linked to Hong Kong criminal organizations. This lineage establishes her as a significant presence, influencing gang dynamics throughout Tokyo. Her background remains initially concealed from others, generating tension upon its revelation.

She relies on her own survival instincts and self-sufficiency, evidenced by independently escaping captivity and navigating hazardous urban landscapes. Her actions include securing food and shelter while actively evading capture, showcasing practical resilience. Discussions with Kai involve clarifying past events, specifically the circumstances of Fujio's death. Sunmi initially believed Fujio died by suicide; Kai corrects her, explaining Fujio perished in a crowd-related accident at a train station.

Physically, Sunmi closely resembles Fujio, Mera's deceased girlfriend. This likeness triggers Mera's memories of his past relationship, amplifying his emotional vulnerability during their encounters. Mera takes her to a seaside motel tied to his personal history with Fujio, further intertwining Sunmi with the characters' shared past.

Her concealed identity as Daishisai's granddaughter becomes a pivotal plot point upon its discovery by Buppa. This revelation causes significant panic within his organization due to potential retaliation from her powerful grandfather. It escalates existing conflicts, with Skunk attempting to exploit the situation by negotiating directly with Daishisai to trade Sunmi for control of Tokyo. These events directly impact gang alliances and leadership struggles, particularly within the Wu-Ronz faction.

Narratively, Sunmi acts as a catalyst, revealing character backstories and intensifying gang tensions. Her movements between factions drive interactions among key figures like Mera, Kai, and Buppa. Her powerful familial connections introduce external forces that reshape Tokyo's gang hierarchy.