Kazuki Hoshino is the younger brother of the protagonist, Ichika Hoshino, in the two-part anime film Gekijōban Collar × Malice Deep Cover. As a child, he excelled in everything he did, which led his parents to favor him over his older sister. They placed their expectations on him to one day succeed the family business, creating a point of tension in the siblings' relationship. Kazuki himself dreams of becoming a musician, and he resents that his sister was able to pursue the career she chose for herself as a police officer. This resentment and jealousy have caused him to be cold and distant toward Ichika, and he believes she does not care about his own aspirations. Despite this strained and estranged dynamic, Kazuki still harbors genuine care for his sister, though he refuses to openly show his affection. In the story, his role is that of a supporting character, with his connection to Ichika serving as an important element of her personal life and backstory.