Posing as Mitsuki Sanada’s brother at school, Kazuki is overwhelmed by male students vying for her attention, leading to tension when he accepts their love letters. Mitsuki, annoyed, assigns him to respond to each one. When D moves into their home, Mitsuki builds a wall to separate her from Kazuki, frustrated by D’s preference for his instructions. A mix-up with Mitsuki’s diary escalates tensions, but a rainy search brings them closer. Yayoi’s arrival as their neighbor complicates their daily routine, testing their group dynamic.