Kanade Amakusa serves as the stern student council president at the protagonist's school, instantly recognizable by her long black hair, sharp eyes, and severe expression. She constantly carries Masamune, a large wooden sword she wields freely to enforce rules and punish wrongdoers.

Her defining drive is a fanatical devotion to "justice," manifesting as strict rule adherence and harsh punishments that make her widely feared. She maintains a cold, stoic, and aloof exterior, rarely showing emotion. Yet beneath this lies buried loneliness and a fear of being hurt again, rooted in severe childhood bullying. This trauma forged her belief that strength and unwavering justice prevent suffering. During that time, she formed a bond with the protagonist, Kanade Oukawa, making a childhood promise to become strong and protect each other. She later misinterpreted his withdrawal as a betrayal, fueling her harshness towards him and intensifying her rigid justice as a shield.

Her development is triggered by the "Absolute Choice" phenomenon affecting the protagonist. Initially dismissive and punitive towards the absurd choices, repeated exposure and forced participation in their consequences begin to fracture her stoicism. Witnessing the protagonist's struggles and the genuine bonds forming within the group challenges her black-and-white worldview. She becomes increasingly involved with the group, particularly the protagonist and her fierce rival, Chocolat. This forces her to confront her vulnerabilities and hidden feelings, sparking unfamiliar emotions like jealousy and confusion, especially concerning the protagonist and Chocolat. These interactions reveal a suppressed romantic affection for the protagonist, stemming from their childhood connection.

Gradually, her rigid concept of justice softens. She starts understanding nuance and values personal connections over blind rule enforcement. Protecting her friends, especially the protagonist, becomes a stronger motivation than abstract justice. This culminates in a critical confrontation where she openly confesses her long-held romantic feelings for the protagonist, directly challenging her former persona and embracing vulnerability.

Her rivalry with Chocolat evolves from pure antagonism into a complex dynamic of competition, mutual respect, and reluctant cooperation, especially against common threats or shared goals involving the protagonist. By the narrative's conclusion, she achieves greater balance. While retaining a strong sense of justice and wielding Masamune, she integrates compassion, acknowledges her feelings, and values her friendships, marking substantial growth from her initially isolated and rigid self.