Aika Takamura is one of the two main protagonists of the series. She is the daughter of Misaki Takamura and becomes the new stepsister of Kōta Takamura after her mother marries his father, Yoshiyuki Takamura.

Aika is characterized as a gyaru, a fashion and lifestyle subculture often associated with a glamorous, outgoing, and rebellious attitude. Her most defining trait is her stark dual personality. In public, particularly in front of her parents, she acts like a perfect angel, appearing kind, refined, and well-mannered. However, once they are alone, this facade drops completely, revealing a much brattier, cheeky, and abrasive side. This hidden personality is described as high-energy, rude, and completely opposite to her stepbrother's straight-laced nature.

Aika's primary role in the story is as the catalyst for the central conflict and relationship. Her arrival disrupts the orderly life of her stepbrother, Kōta. Initially, their relationship is one of constant arguments and clashing personalities, with Aika's free-spirited chaos frustrating Kōta's nit-picky and responsible demeanor. The narrative follows the evolution of this tense dynamic.

Her motivations shift as the story progresses. In the beginning, she seems to take pleasure in antagonizing Kōta and doing whatever she wants, including bringing other boys home, which drives him crazy. However, after a certain incident, her behavior changes, and she begins to develop a fondness for her new stepbrother. This fondness grows into a more intimate and sexual relationship, with Aika seeking physical affection and closeness from Kōta. The series suggests that beneath her wild exterior, she is drawn to Kōta's caring and responsible nature, which provides her with a sense of safety and stability. A traumatic event from her past, described as an assault, is revealed to be a key reason for her need to feel safe and held, a need that Kōta fulfills.

Key relationships define Aika's character. Her relationship with her stepbrother Kōta is the central focus of the series, evolving from animosity to a complex romantic and physical bond. With her mother, Misaki, and stepfather, Yoshiyuki, she maintains a perfect, angelic image, highlighting her manipulative and performative side. Information regarding her relationships with any other characters, such as friends or peers, is not detailed in the source material.

Aika shows notable character development from a seemingly shallow and antagonistic figure to a more vulnerable individual with genuine emotional needs. While she starts as a "full-blown teenage disaster" who is "super-bratty," she gradually reveals her insecurities and the reasons behind her behavior. Her growing attachment to Kōta is not merely physical; she craves intimacy and a deeper connection, demonstrating a capacity for emotional vulnerability that contrasts with her initial abrasive persona. The series also explores her ability to torment and tease Kōta, using his own restraint to her advantage as their relationship deepens.

Regarding notable abilities, Aika is portrayed as a regular human character with no supernatural or extraordinary powers. Her abilities are social and interpersonal. She is highly skilled at maintaining a false public persona, effectively deceiving her parents. She is also adept at manipulating situations and provoking reactions from Kōta, driving the comedic and romantic tension of the story.