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

Amane Fujimiya lives alone in a messy apartment—right next door to Mahiru Shiina, a veritable "angel" who's the smartest, prettiest girl at school. When a chance encounter on a rainy day draws them together, Mahiru finds herself spending more and more time helping Amane get his life in order. And though they both insist they have no intention of dating the other, slowly but surely, something more than friendship begins to blossom. (Source: Square Enix)