Kemeko Deluxe!, Gokukoku no Brynhildr Recommendations
Both have the same situation where the guy and girl met once when they where children and after sometime they where able meet again.
Both have the same school settings.
Both have the same sci-fi, romance, harem and action genre.
Both have similar situation where girl is running away from an evil organization.
And, both have the same predicament where guy got involved.
One has an extra comedy genre, while the other one has a mystery genre.
The girl in one anime is an ET, while the girl in the other anime is a gov't science project that got away.
Both have the same situation where guy meets girl, then guy loses girl only to come back not remembering who the guy.