Kono Danshi, Uchuujin to Tatakaemasu.

Earth has been invaded by aliens, and humanity's supposedly specialized countermeasure consists of only three people who seem woefully unprepared to save anyone. Their fortunes change when Arikawa discovers an amnesiac boy named Kakashi, the only person known to possess the power needed to defeat the invaders.

Kakashi can fight, but he has no memory of who he was, no understanding of why this ability belongs to him, and only a broken phone connecting him to his lost past. As Arikawa and the severe Shiro become his makeshift family, the pressure of being humanity's sole weapon forces Kakashi to confront a question more frightening than the aliens: if everyone only needs his power, who needs him as a person?