The main character's best friend confesses to him when he turns 20 and pushes quite forcefully for their relationship to progress beyond friendship. It turns out that they made a childhood promise to each other that the main character forgot about but the best friend never did and made long-term plans for them to be together forever. Seme/best friend is quite possessive but it's ok because this is a cute manga lol
Aruhi Hyouhenshita Shinyu Ni Kokuhakusa Remashita