Ginga Onimaru is Nasa Yuzaki's cousin, adopting distinct yakuza-inspired speech patterns in daily life. He refers to school as "streets" or "turf," sports meets as "gang wars," classes as "gangs," and pop quizzes as "how to kill during raids." He calls his teacher "boss," chores "raids," kitten milk "white powder," and toys "blasters," frequently causing misunderstandings about criminal ties. His consistently aggressive tone reinforces this perception.

Physically imposing, Ginga is notably tall and maintains a serious expression to project toughness. He typically wears elegant suits, creating a stark contrast with his student status. His initial encounter with Tsukasa occurs at the Arisugawa bathhouse; his suspicious demeanor and direct approach lead her to suspect he is actual yakuza. Tensions rise until Nasa arrives and identifies him as family. Ginga later recounts how Nasa supported him during middle school, though his word choice obscures the sentiment. Intrigued by Nasa and Tsukasa's relationship, he demands proof of their genuine love through a public kiss—a request Tsukasa initially resists due to embarrassment. Ginga ultimately validates their bond without requiring the kiss.

A compassionate side emerges when Ginga enlists schoolmates and a teacher to transport a stray kitten found near school. Citing the incongruity of a "yakuza" caring for the animal, he entrusts it to known cat lover Nasa. His storyline intersects with Shirogane Yaiba, who develops romantic feelings for him. Discovering he is being stalked, Ginga corners the stalker in a confined space. Shirogane uses this moment to confess her feelings, though the surrounding altercation remains ambiguously described. His response to her confession is enigmatic.

Ginga expresses fierce loyalty to family, declaring to Tsukasa: "If you're his wife, you're my sis!! I'll protect you with my life!! But not if your love ain't real." His name "Ginga" (銀河) translates to "galaxy."