Yūji Kōsaka is the younger brother of Tamaki Kōsaka and a classmate of protagonist Takaaki Kouno. Despite his wealthy background, he flaunts a carefree and frivolous attitude, frequently flirting with female students. He primarily acts as the "perverted friend," his persistent pursuit of girls often sparking comedic clashes with his sister. Tamaki retaliates against his behavior with physical reprimands, usually her signature painful forehead-crushing "iron claw."

He harbors noticeable jealousy towards Takaaki due to the latter's perceived popularity with female classmates, often resenting Takaaki for "taking" all the girls. This envy fuels interpersonal tensions within their group. Despite this, Yūji maintains a close friendship with Takaaki, regularly appearing alongside him during school activities and commutes.

Photography is a prominent hobby, though he wields his camera primarily to capture female classmates rather than artistic pursuits. This habit sometimes intensifies conflicts with his sister and others. His unlucky streak and comedic suffering are recurring traits, with Tamaki's physical interventions forming a running gag.

In the *adnext* OVA, his role involves eavesdropping on a conversation where classmate Manaka Komaki addresses Takaaki by his first name outside the library. Yūji then spreads this detail among their classmates, triggering varied reactions from female characters around Takaaki and fueling a broader comedic misunderstanding. This action reflects his established tendency to instigate or amplify social situations as a catalyst for lighthearted conflict.

His portrayal remains consistent across official media, including anime seasons, OVAs, or spin-offs, with no significant character development or alterations to his background. He consistently provides comic relief through his envy, romantic misadventures, and misfortunes stemming directly from his personality traits.