Season 1: Amane lends Mahiru his umbrella, and in return she starts cooking and caring for him. At Christmas, Itsuki and Chitose visit and discover that his neighbor is the Angel. Mahiru shows flashes of possessiveness at the New Year's shrine visit. On Valentine's Day she gives Amane hot chocolate but loses her nerve and goes home, and Amane later finds chocolates and a card from her at his front door. In a key emotional turn, Amane finds a haggard Mahiru talking to her mother. He invites her in, and while he holds her hands she reveals that her parents never really cared for her, despite all her efforts to excel.
Season 1 finale: At the sports festival, Mahiru uses a game to declare publicly that Amane is a person very dear to her. Amane overcomes his cowardice, and by the end of the season the two are together as a couple.
Season 2 (2026, directed by Chihiro Kumano) continues their story as a couple. The sources found didn't give detailed plot points for it.
Hidden meaning: Looking back, the "spoiling" runs both ways. Mahiru takes care of Amane's daily life, but Amane gives her what she really lacked: someone who values her for who she is.