Ginga Onimaru is a student and the cousin of the protagonist, Nasa Yuzaki, making his introduction during the events of the high school arc. His most defining characteristic is his manner of speech and behavior, which heavily mimics the style and vocabulary of a yakuza enforcer. He refers to everyday locations and activities in thuggish terms, calling school the "streets" or his "turf," class a "gang," chores "raids," and sports meets "gang wars." He addresses his teacher as "boss" and even refers to kitten milk as "white powder," a choice of words that frequently leads others to mistakenly believe he is involved in organized crime. Despite this intimidating and aggressive facade, Ginga is pure of heart and possesses a strong sense of honor. He genuinely enjoys helping strangers and protecting people, and he respects the privacy of others.

When Ginga first appears at the Arisugawa bathhouse looking for Nasa, his suspicious demeanor immediately puts Tsukasa on edge, leading her to believe he might be a real yakuza. The tension is only diffused when Nasa arrives and reveals that Ginga is his younger cousin. Ginga holds deep respect for Nasa, who acted as a reliable mentor to him during middle school, though he couches descriptions of Nasa's help in his usual menacing slang. Intrigued by Nasa and Tsukasa's relationship, Ginga becomes a skeptic who demands proof of their love's authenticity. He challenges the couple to prove their bond by kissing in front of him, a request that embarrasses Tsukasa and excites Nasa, though Ginga ultimately validates their relationship without requiring the act.

A notable display of his hidden kindness occurs when he brings a stray kitten to Nasa. Ginga has enlisted his schoolmates and teacher to transport the animal, explaining that it would be too strange for a "yakuza" like himself to be seen caring for a kitten and that it would be better left with a known cat lover like Nasa. In his own romantic subplot, Ginga attracts the affection of a girl named Shirogane Yaiba. He becomes aware that someone is stalking him and manages to corner the pursuer. In a chaotic moment where he confronts the stalker in a dark, confined space, Shirogane takes the opportunity to confess her feelings for him. Ginga's response to her confession remains characteristically enigmatic. Throughout the story, he shows fierce loyalty, declaring that if Tsukasa is truly Nasa's wife, she is like a sister to him and that he would protect her with his life. Physically imposing, Ginga is very tall and maintains a serious expression to project toughness, typically wearing an elegant suit that creates a stark contrast with his status as a high school student.