Otonari no Tenshi-sama ni Itsunomanika Dame Ningen ni Sareteita Ken

Amane lives alone in an apartment, and the most beautiful girl in school, Mahiru, lives just next door. They've almost never spoken-until the day he sees her in distress on a rainy day and lends her his umbrella. To return the favor, she offers him help around the house, and a relationship slowly begins to blossom as the distance between them closes...

(Source: Yen Press)