Split personality trigger

So I have this character who is a vessel for a four-way Split personality. Each one is female and each one has control over one of the four classical elements with the personalities to match.

My problem is I'm trying to figure out how to trigger a change between them. Will provide necessary details if asked.

Well, speaking from experience, DID-cases tend to "shift places" whenever the stress becomes too much for the current personality to handle, prompting the shift and allowing the stressed out persona a "time-out" while the unstressed one take charge to resolve the stressful situation at hand.

The four personas of your character are "Water/Air/Earth/Fire"-aligned, I can presume?

In that case, my suggestion would be to base the stress-trigger on the situation, rather than the specifics of it.

Ask yourself "Which one would be best suited to deal with this mess?"

Stress born from a physical threat? Send out "Fire" and burn the threat away.

Stress born from every day life? Send out "Earth" and calm down like a mountain.

Stress born from worrying over nonsense? Send out "Air" and just float away from it all.

Stress born from getting stuck on one specific thing and feeling frustration over it? Send out "Water" and move on.

There is also the question of dealing with the memory gaps whenever a persona shift as well, since they might share memory and they might not, but safe to be certain, if persona A shifts into persona B, then B is not going to remember what they did the first half of the day, which might cause a bit of trouble if Persona A promised something to someone else before the shift.

Depending on how broken the main character with the DID are (as this comes from suffering a trauma in the early childhood, and tend to stick around for life unless treated...) then there is the possibility that the different personas might have developed their own "tell", which might tip off to others "who" is in charge for the moment, if anyone have learnt about it (but don't expect others to actually believe the character in question have DID, since the case I'm familiar with have tried to tell others, but been brushed off as nonsense and gone untreated despite their best efforts to get help.)

Things like one of them tucking a strand of their hair behind their ear or talking with a slightly different accent or straighten their back when they walk... small things like that.

Hope this was helpful...

It is very useful, but I still need to shed some light on a few important details.

1) I meant vessel literally, they're not personas but actual peoples minds in one body. 2)The identities are not aware of each other.

Good. clarity.

1) The stress-factor can still serve as trigger, since this is a Mind Hive working on the basis of one brain. Meaning that if the brain experiences enough stress, it will change out like with a DID case and let one of the other ones out, regardless of the source for that stress so to say.

2) Memory gaps are definitely present then, since they don't share information and they might be a bit confused over who it is that keep changing their clothes when they're not "looking". They might learn how to recognize "their" home by having woken up in it often enough, and "familiar" people from repeated interaction with them. If they suspect something is up, one of them might start a "Post-It system" meaning that they leave a post it with a simple message for whoever takes their place, and encouraging the next one to write something on the same note. This way, the four might be able to communicate with each other and learn of the others' existence, but not what kind of people they are, unless they're willing to write four diaries just to keep the others updated on what's going on while they're gone in the dark.

Does this help as suggestions?

It is very useful, thanks. Would you perhaps like to hear who the four people are?

Oh, I would love to.

By all means, continue.

Rose is the one who controls air,she's cute, carefree, and very friendly.

Samantha is the one who controls fire, she's brash, rude, and blunt. At the same time she's quite passionate.

Elizabeth controls earth, she's the most mature of them all and the most, pardon the pun, down to earth.

Lastly there's Heather who controls water. She is melancholy, reclusive, and the most pessimistic.