I am looking for something that involves the main character having an alter ego that is completely different from their usual personality. The only manga I can think of that is like this is Yurara no Tsuki. Thanks!

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Ao no Fuuin
Nurarihyon no Mago
Othello <-- this one is perhaps closest to a real alter ego, the others are more like being possessed by spirits
Ryou

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Reading: Kingdom, Sangokushi, Historie

DNangel

i know the anime of chaos head has a dude with one...
hitman reborn - mukuro rokudo. his well... something lives in this girl kinda giving her multiple personalities

not really action but:
ageha 100%
momoiro heaven
nousatsu rock shounen

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Thanks! These look interesting. 😛

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Some of these threads might help you too:

http://www.mangaupdates.com/showtopic.php?tid=9717
http://www.mangaupdates.com/showtopic.php?tid=5144
http://www.mangaupdates.com/showtopic.php?tid=3336
http://www.mangaupdates.com/showtopic.php?tid=4880
http://www.mangaupdates.com/showtopic.php?tid=6413
http://www.mangaupdates.com/showtopic.php?tid=10808
http://www.mangaupdates.com/showtopic.php?tid=10174

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