Stella, an orphan from an English orphanage overseen by Sister, traveled alone to Japan to reunite with him, driven by deep attachment formed under his care. Her fierce protectiveness toward Sister manifests as immediate hostility toward Maria, whom she perceives as a rival threat.

She presents as a small, blonde-haired, blue-eyed girl with an innocent demeanor that initially endears her to others, including Kou Ichinomiya—who finds himself unexpectedly fond of her despite general difficulties with children. She addresses Kou as "Big Brother," though the term implies hierarchical dominance rather than familial affection.

Beneath this exterior lies a dominant persona. When provoked, she undergoes a physical transformation, growing to 260 cm tall and 210 kg with a heavily muscular build. Her speech shifts to a Hiroshima dialect, adopting a tyrannical attitude to assert control. She challenges Kou to a "Fight Club" session, overpowers him effortlessly, and declares ownership. The Iron Brothers recognize her authority, calling her "Boss" and serving as loyal underlings.

This duality reflects coexisting aspects: one gentle and childlike, the other militant and commanding. Sister personally trained her in combat, endowing her with formidable tactical instincts and physical prowess that challenge experienced fighters. She views herself as his devoted disciple, bordering on adoration. Though harsh toward most, she interacts relatively normally with the Iron Brothers while exploiting their gullibility.

Her relationships center on unwavering loyalty to Sister. With Kou, she oscillates between exploiting his naivety and enforcing hierarchy after defeating him. Under the bridge, she asserts dominance over community dynamics, commanding the Iron Brothers and confronting newcomers like Maria.