Akane Hiyama is a central character and the primary female lead. She is a high school student who is widely regarded as the school's idol, known for her striking beauty, long red hair, and red eyes, standing 163 centimeters tall with a birthday on October 28th. A member of the athletics club, she maintains an outward appearance of being gentle, polite, and charismatic, which has earned her significant admiration from her peers. However, this friendly and composed exterior is largely a social facade. In truth, Akane has very little interest in people or matters that do not concern her primary focus: Seiji Aino.

Beneath her public persona, Akane possesses an intensely obsessive and possessive personality, fitting the archetype of a yandere. She is completely consumed by her love for Seiji, which she has harbored for some time, having noticed his admiring gazes long before the story begins. This devotion manifests as extreme jealousy and a hair-trigger temper whenever she perceives another female as a threat to her relationship with him. She is known to refer to potential rivals dismissively as "females" and will not hesitate to resort to violence to eliminate what she sees as a danger to her romance. Despite this volatility, her motivations are rooted in a genuine, if dangerously expressed, desire for Seiji's affection and a deep-seated wish to build a future with him. She diligently practices "bride training," honing domestic skills like cooking and housekeeping with the specific goal of becoming his ideal wife.

Akane's role in the story is set in motion by the intervention of the angel Guri and the Kiss Note. Although Seiji initially had a crush on her from afar, Akane is revealed to reciprocate those feelings with an intensity he never expected. Her violent reaction to discovering Seiji kissed Guri is what first exposes her true nature and thrusts her into the chaotic central conflict. Throughout the series, she acts as both a dedicated lover and a fierce protector of Seiji, often creating as many problems as she solves due to her possessiveness. Her key relationships are defined by her singular focus. Her bond with Seiji is the axis around which her entire life turns, and she is fiercely loyal to him. She views Guri, whom she nicknames "little monkey," as a primary rival and refuses to acknowledge her as anything other than an obstacle. Yuzu Kichougasaki is her younger half-sister, but Akane's priority remains Seiji, leading her to treat Yuzu with a kind but distant cordiality rather than deep familial affection. In contrast, she is notably warm and welcoming towards Seiji's younger sister, Akua, as she envisions Akua becoming her future sister-in-law.

Over the course of the narrative, Akane undergoes development from a violently obsessive stalker into a more stable, though still possessive, partner. While her aggressive tendencies, especially towards rivals, never fully disappear, she learns to coexist with the other girls in Seiji's life, eventually agreeing to a polyamorous arrangement. This growth shows her love maturing from a purely selfish and destructive force into something that, while still extreme, allows for a degree of compromise. Her most notable abilities are her formidable combat skills and superhuman physical prowess. She is almost always seen wielding or throwing an endless supply of kukri knives, which she conceals on her person. When motivated by her love or jealousy, she displays strength, speed, and durability far beyond that of a normal human, capable of feats such as defeating a Cerberus and throwing its body a significant distance. Furthermore, she gains immortality through a blessing from Guri, making her unable to die or suffer lasting injuries, which only emboldens her aggressive behavior.