The main character in Parasyte changes personalities completely when

Kuro from Kurozuka changes personalities often [though not 180 degrees], and I guess you can consider the catalyst traumatic enough.

Elijah from [m]Eden: It's an Endless World[/m].

Shut Hell
Black Cat
Ubel Blatt
Gakuen Ouji -- I think the main girl was acused of stealing her friend's boyfriend which turned her into a weirdo who hides her beauty, though I can't really remember.
Claymore
Cavalier of the Abyss -- sequel to Immortal Regis
Bengo!
En Passant
Embalming - The Another Tale of Frankenstein- regarding the second set of protagonists (whose names I forget), the guy and the young girl frankenstein. The guy was changed after the girl died and the girl was changed by, well, being turned into a frankenstein.
Kimi no Sei
The Snow Queen
Vinland Saga
Full Metal Alchemist - maybe after thir mother dies. Or even after the failed transmutation.

I didn't really feel like checking so sorry if anyone else mentioned any of these already.

Real there are three main character ( even the minor character ) in this manga. They all have accidents that change their life



Try it. It's Really good.

...! Thanks, everybody! And esp. magical_poof! That's a lot for me to work through! (But I'd welcome even more, lol. ) Thanks again.