Ouchi de Koi ga Matteru

Warm and bittersweet, a temporary life under the same roof. Ui is a third-year high schooler cramming for exams who ends up moving in with distant relatives: a kind grandmother and her grandson Rintarou, a first-year who acts cool but is secretly a grandma's boy. As they live together, something warm stirs in Ui's heart. Meanwhile, Rintarou finds himself increasingly caught up in Ui and her carefree ways...!?

(Source: Kodansha, translated)