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I need some help. Trying to remember an anime I watched when I was younger.

All I remember from it is this--- there's a boy he's still in school. Can transform into a girl to fight monsters. The girl is more or less just using the boys body to fight since something happened to hers. and she can control the boys movements example: she made him grab his crushes hands and pulled her close. Oh one more thing I remember, there was a 2 or 3 headed ooze monster. it was like a big blob with 2 or 3 heads that to me kinds looked like dogs or gargoyles. The girl took over the boys body and transformed to fight it. PLEASE if anyone has any idea what I'm talking about please say something. I haven't seen that anime in years. I just can't seem to remember anymore about it or what it's called. Thank you!
birdy decode is an older anime and birdy is a girl who works for the interspace police force or something but posses a boys body to fight aliens while she's on earth..
a good site to look up and find anime is
birdy the mighty decode might be it not 100% also kampfer is another anime where a boy turns into a girl to fight.
Guilty Crown? Idk if it's newer or not and I'm not that far in so could it be that? I dunno, I haven't watched it in a while
@Blew thank you :)
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