Kyoudai no Rule.

Mikage Aiba, a high school senior, has a younger stepbrother named Tomoki. Mikage values rules and structure, while Tomoki is more relaxed. The two have maintained a close relationship since their parents remarried 12 years ago. They follow three key rules: treating each other like family, setting time limits, and not crossing the final line. Their dynamic is disrupted when Mikage is accepted into university, causing tensions to rise and their established order to break down. In response, they establish a new set of rules to navigate their changing relationship.