Kyosuke Wakao is the central protagonist of the adult anime series Step Sister. He is a young adult male with brown eyes and brown hair cut to approximately ear length. His life is dramatically altered following the death of his father, Kazuma Wakao, a renowned painter who leaves behind a substantial estate, including a collection of valuable paintings.

At the outset of the story, Kyosuke is driven by a deep-seated resentment toward his stepmother, Megumi Wakao. He blames her for his own mother's death and strongly disapproves of his father's marriage to a much younger woman. Inheriting everything after his father's passing, Kyosuke exercises his authority by ordering Megumi to leave the house. However, a practical obstacle presents itself: the large property tax on the inherited paintings, which Kyosuke wishes to keep. This financial pressure becomes the context for a morally fraught negotiation with his stepmother.

Kyosuke is characterized by a manipulative and strong-willed nature, with a pronounced sadistic streak that he discovers and embraces through the story's events. His primary motivation shifts from simply expelling his stepmother to entering a Faustian bargain that allows him to explore his darker impulses. Megumi proposes a deal where she and her daughter, Yuna, Kyosuke's step-sister, may remain in the house in exchange for Kyosuke being granted unrestricted freedom to do whatever he wishes with Yuna. Kyosuke accepts this proposition, viewing Yuna as a perfect submissive counterpart to his own dominant and sadistic desires.

In his role within the story, Kyosuke acts as the catalyst for the narrative's central conflict and its psychological exploration. He is not a passive heir but an active agent who uses his inheritance as leverage to gain control over his stepfamily. The deal he makes sets in motion a dark and intensive relationship with Yuna, who, desperate to preserve the illusion of a family unit, agrees to his demands. Kyosuke's actions force both Yuna and eventually Megumi to confront and develop their own masochistic preferences, creating a twisted family dynamic born from his initial ultimatum.

Kyosuke's key relationships are defined by power and transaction. His relationship with Yuna evolves from one of blackmail and sadistic control to a more complex, albeit still dysfunctional, bond where she comes to desire her role. His relationship with his stepmother, Megumi, is also adversarial and manipulative, as she is an art dealer with her own agenda regarding his father's paintings, leading to a battle of wills that eventually takes on a more intimate and intense form. Through these interactions, Kyosuke undergoes a form of development, not towards redemption, but towards a deeper self-awareness of his own cruel and sadistic personality. He acknowledges the immorality of his actions, yet continues down that path, finding a strange fulfillment in the power he wields over the two women. His notable abilities lie not in physical prowess but in psychological manipulation and his cold, calculating approach to exploiting the vulnerabilities and desires of those around him.