 If you liked
Busou Renkin
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 ...then you might like
Yuusen Shoujo: Plug-in Girl
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Yuusen shoujo is about a guy who saves a girl and they both require new "bodies" (won't give anything away), they meet later on in life and the story begins from there.
Both series have a cheerful male protagonist who go through something hard and gains new power. The styles of the series are in general very similar! Busou renkin is more of a mainstream fighting manga though. Yuusen shoujo focuses a bit more on the character development and their surroundings. I love both series pretty equally, but more non-typical shonen fans Yuusen shoujo would be the better choice.