 If you liked
Kiseijuu
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 ...then you might like
Aflame Inferno
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I honestly can't believe these two weren't recommended for each other when i was reading one i thought "wow these two are rather similar"
both have high schooler sharing body with something not human, both fight their own non-human kind both main characters gain power while merging with each other gradually both of the main characters start to see the world differently and feel non-human their personalities seem to change and both have subtle romance