 If you liked
Tetsuwan Birdy
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 ...then you might like
Kyoushoku Soukou Guyver (2005)
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Format is very similar, Unassuming boy gains a super powered alter ego due to alien influence and battles various monsters, while discovering the bigger picture. Guyver is just darker and more violent, also instead of the main character turning into a Woman, he turns into biological battle armor.