 If you liked
Kono Minikuku mo Utsukushii Sekai
|
 ...then you might like
Narue no Sekai
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In both series, the main male protagonist meets a strange girl who seems to not be from Earth. From there on, they have a strange relationship that turns from friends to romance.
Both series has science fiction themes although Narue no Sekai has a more lighthearted mood. They both also take place in a school life setting filled with comedy and drama.
Both series' main male protagonist also has similar personalities.