Amagami to Hakuchuumu

Michishita, an introverted gay architect, has a crush on his mysterious apartment neighbor, Kase. Kase always hides his face behind a mask and rarely makes conversation with anyone, leaving Michishita with no intention of pursuing his unrequited love, content to just admire him from afar. One day, Michishita falls victim to a serious accident, only to be saved in the nick of time by Kase. However, Kase gets seriously injured while shielding Michishita and is taken to the hospital in critical condition, unconscious. When Kase finally awakens, to Michishita's surprise, he starts calling him "big brother" and begins acting adorably toward him...?

(Source: Kaiohsha, translated)