Here's the gist of it. (yes im easily influenced by series)
i was thinking of a dwarf monk

raised by monks after some traumatic happening i still need to flesh out. its a group of monk who teaches peace and try to solve most things through non-combat.
my little dwarf was of course mentally ruined by the previous event and being a dwarf of battle rager decent have issues controlling his rage in combat.
The monks in time did manage to put a lid on his more violent emotions and he mainly follows a at times overly optimistic.. "cant we just talk things out?".. that tries not to kill if it can be avoided...

but violence directed against my dwarf does poke at the lid and at times it comes out and gets the mentality of a battle rager (or close to it).. things are to be solved by a fist through the face.. or a boot up an ass..

the upcoming GM (my wife) is not opposed to it.. but would like some ideas on how to trigger the change.. and how to change back..
I would recon some sort of saving throw when damage is inflicted on the character... people die around him.. intense battle.. something of the sort
after said change there would be a need to calm the character (the other players might help out... or impede if it would happen that way)
getting away from the battle... resting.. meditating would of course help

so any interesting suggestions? :)

Do you mean that he has Dissociative Identity Disorder? Schizophrenia is characterized by hearing voices that may offer advice, or give orders, but never assume control. Dissociative Identity Disorder is when there are multiple distinct personalities and any one of them could be in control at any given time.

Assuming you mean DID, you need to decide how often you want this to occur and how detrimental you want it to be. Maybe it's triggered only by death, maybe it's triggered when someone falls unconscious, maybe it happens every time a friend is bloodied. How specific, how often, and how detrimental it ends up being are all up to you and the DM, but I would let the other players in the group know too. That way you aren't surprising them with a character that doesn't always play for their team.

true.. i tend to get those mixed up sorry.. probably due to schizo being used badly in movies and such :)
but yes.. DID would be the correct thing.

Luckily both persons will not activly work against the group.. its not something as extreme as a paladin going chaotic evil suddenly.
its probably going to look more like a change from Fredegar Rockcrusher  into Thibbledorf Pwent.. both in general on the good(ish) side of things.. but their problem solving abilities have completely different angles

as seen in the iron fist season the DID triggers tend to not be very specific.. its more the state of mind of the person, stress makes one of them come out while things that the person finds calming brings the other one forward..

perhaps something the level of violence have different chances of bringing forward one personality?

for the group we tend to enjoy some strange setups.. so in most cases we simply go.. "dont make anyone evil" and see what happens
we've had a party of 6 where there was 4 different kinds (luckily) or arcane casters (im trying for a necromancer thats not evil... though i have some major mental gymnastics when i in the end can create undeads... one plan is to get a hold of something that allows me to speak with dead.. to basically ask permission :P