I once read this manga a while back and it was about this highschool girl who for some reason( due to a health problem or her just being that way) acted a lot younger, like a kid. And she had a friend( he was a little boy) who was should've been the one acting like the kid, but for SOME REASON( due to ANOTHER HEALTH PROBLEM, OR JUST THE WEIRDNESS OF MANGA) acted a lot older than he was, like a high schooler. So the boy was always getting the girl out of trouble that she got into and "babysitting" her. I really loved the whole originality to it, so please answer! ^^

Reminded me of the summary of Recorder to Randoseru, although they are siblings, and there seem to be no releases out, but there is an anime.

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"I'll shut your mouth~~~~~ with mine~~~"

二息歩行

i know what manga u're talking about, cuz i already read its description here on mangaupdates, but saddly i dont remember its name now! sorry!

http://www.mangaupdates.com/series.html?id=76745

this?

I've never read The Friendly Winter, so I don't know if it's actually it, but the roles are switched (the girl is the mature one in a younger body and vice versa).

ahh....this was probably really unhelpful....