Jun Isurugi is a supporting character known for his striking appearance and his deeply protective, almost singular focus on his younger sister, Noe Isurugi. As a third-year student at Hotarukawa High School, he holds the position of ace and captain of the basketball team, wearing the number four jersey. His good looks and athletic prowess make him quite popular among the female students, but he remains largely indifferent to their attention, with his world revolving almost entirely around his sister.

Jun and Noe share a difficult family background, having lost their father and grandmother when they were young. With their mother frequently away for work, Jun took on the role of primary guardian. This responsibility has forged a strong sense of duty in him, and he is often seen caring for Noe's needs and watching over her well-being. Since they attend different schools, he regularly drives her back and forth on his motorcycle, a visual representation of the bond that defines his daily life.

Personality-wise, Jun presents a calm, mature, and responsible exterior. He is seen as the classic "cool and collected" upperclassman, yet this facade is specifically tied to his role as a devoted brother. His motivations are consistently rooted in his desire to protect and provide for Noe, to the point where his dedication appears to surpass that of a typical sibling. This intense focus leads him to take actions that significantly drive the story’s romantic conflicts, revealing that underneath his serious demeanor, he harbors deep, possibly romantic feelings for his own sister, a fact that causes him to momentarily break his stoic composure when teased about it.

In the story's central complications, Jun serves as a catalyst for the main character Shin'ichiro Nakagami. After the female lead Hiromi Yuasa falsely claims that the person she likes is the basketball ace at Hotarukawa High School, Jun approaches Shin'ichiro with a proposition. He asks Shin'ichiro to date Noe, believing it will make her happy. Frustrated by this request, Shin'ichiro retorts that Jun should instead date Hiromi. In a surprising move, Jun agrees to this arrangement and enters a relationship with Hiromi, despite having no genuine romantic feelings for her, further entangling the characters in a web of misunderstandings.

Key relationships define Jun's role entirely. His bond with his sister Noe is his most dominant characteristic. His relationship with Hiromi is a convenient construct born from a mutual agreement rather than genuine affection, though the arrangement complicates the lives of everyone involved. His interactions with Shin'ichiro are often tense, stemming from a rivalry of sorts over who holds the key to Noe's happiness.

As for his development, Jun begins as a figure who appears perfect and composed but is ultimately driven by an obsessive devotion to his sister. He is willing to manipulate the feelings of those around him to secure Noe's happiness. By the end of the story, Jun does not find a traditionally happy resolution; he ends on a more negative note, having lost the object of his affection. However, he emerges from the experience changed, having been forced to confront the complicated and unsustainable nature of his feelings. His notable abilities include his skill in basketball and his proficiency with his motorcycle, though his most defining ability is his capacity for self-sacrifice and emotional manipulation in the service of his sister’s well-being.