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)