Asuka Matsubara is a character introduced in Student Council's Discretion Lv. 2, where she appears as a significant figure from the protagonist Ken Sugisaki's past. She is his former girlfriend and childhood friend, having grown up as his neighbor. Physically, Asuka is depicted as a teenage girl with brown eyes and purple hair that reaches her waist, often styled in a ponytail.

Asuka has a mischievous and playful personality, with a particular fondness for teasing and tormenting Ken Sugisaki, a trait she has displayed since they were children. Her pranks are elaborate and invasive, such as secretly installing approximately thirty wiretapping devices in Ken's room to listen to his conversations. This constant needling has, as a side effect, made Ken highly adept at responding to her tricks. While she enjoys mocking Ken and the student council, she also has a kind side and is capable of accepting others, notably agreeing to be friends with the student council members after they request it. Her competitive nature often leads her to cheat in games to secure a win. Despite her sharp edges, she is described as popular and sociable.

A defining aspect of Asuka's character is her relationship with the Sugisaki siblings. She acts as a sweet and caring elder sister figure to Ken's younger sibling, Ringo Sugisaki. However, this care is expressed in a uniquely troublesome way, as she deliberately teaches Ringo incorrect and vulgar vocabulary, leading the innocent girl to unknowingly insult those around her. Asuka justifies this by claiming that Ringo is too naive and would otherwise be taken advantage of, though she privately acknowledges that she may have taught her a little too harshly.

Asuka's past with Ken is the emotional crux of her backstory. She had a crush on him from a young age and eventually confessed in middle school, leading to a mutual relationship. Their romance ended tragically when Ringo, who also harbored feelings for Ken, became distraught and harmed herself. The resulting emotional fallout caused Ken and Asuka to break up, after which Asuka attended a different school. This painful incident is a key catalyst for Ken's later personality and his desire to form a harem where no one gets hurt, making Asuka the starting point for his transformation.

Asuka reappears later in the story, specifically around the graduation arc, where she reconnects with Ken. She acknowledges that Ken is fundamentally a good person trying to make everyone happy. In a significant moment, she challenges him directly, asking him to look only at her, implying that his pursuit of a harem leaves her own happiness unfulfilled. When Ken resolves to pursue his own unconventional form of happiness, Asuka accepts his change and decides to join his path. Her return creates immediate chaos within the student council, as she easily identifies and exploits the insecurities of the four main heroines with a few cutting words, declaring herself Ken's "unmatched girlfriend" with the goal of destroying the harem he has built. After a confrontation, however, she ultimately comes to fully accept Ken and his companions in the student council.

Asuka's influence extends beyond her direct interactions. She is indirectly responsible for the hostility that the character Kagami Hokuto feels toward Ken. Asuka had befriended Kagami's mother, and Kagami, who feared that his most precious person would follow the same painful path as his own mother, became hostile toward Ken upon learning that Asuka was reconciling with him. In terms of abilities, beyond her social acumen, Asuka's notable skills include her psychological manipulation, her talent for devising complex pranks like wiretapping, and her verbal sharpness, which allows her to easily dishearten her opponents in conversation.