"Ane-Koi" centers on the life of Saitou Kouta, a high school student who finds himself entangled in a complicated relationship with his older step-sister, Saitou Anna. After their parents remarry, Kouta and Anna become step-siblings, though they initially maintain a distant and awkward relationship. Anna, a college student, is outgoing, confident, and often teases Kouta, while Kouta is reserved and struggles to navigate his feelings toward her. Despite their differences, the two gradually grow closer, blurring the lines between familial and romantic affection.

The narrative explores the evolving dynamics between Kouta and Anna as they navigate their unconventional bond. Anna’s playful and flirtatious behavior often puts Kouta in uncomfortable yet intriguing situations, forcing him to confront his growing attraction to her. Meanwhile, Anna’s own feelings remain ambiguous, as she oscillates between treating Kouta as a younger sibling and expressing a deeper, more intimate interest in him. The story delves into themes of love, boundaries, and the complexities of familial relationships, particularly in the context of step-siblings.

Secondary characters, such as Kouta’s classmates and Anna’s friends, add layers to the story, providing external perspectives and occasionally complicating the central relationship. These characters often serve as foils or catalysts, highlighting the tension and emotional stakes between Kouta and Anna. The manga balances lighthearted, comedic moments with more serious, introspective scenes, creating a tone that shifts between playful and poignant.

As the story progresses, Kouta and Anna’s relationship becomes increasingly entangled, with both characters grappling with societal expectations and their own moral dilemmas. The narrative does not shy away from exploring the ethical implications of their bond, presenting their struggles with honesty and nuance. The manga ultimately leaves the nature of their relationship open to interpretation, allowing readers to draw their own conclusions about the characters’ feelings and choices.