Aru Hi Sora kara Totsuzen ni.

Yuuto is an ordinary run-of-the-mill high school student. But one day he gets struck by lightning, and things start to take a turn. A strange person, who goes by the name "Yadzuchi," shows up and starts following Yuuto around. Yadzuchi turns out to be a ghost with mysterious spiritual powers, and proclaims (insists!) that Yuuto is the reincarnation of Princess Aki from a thousand years ago. Now, Yadzuchi won't leave Yuuto alone until he remembers who he really is!

(Source: June Manga)