Now that I see a power for Ichthys, I've a rough idea of the trigger event.
So, to start off, the ratings; Changer, Brute, Blaster, Shaker and Master, though I'd argue it's a Striker instead for that last one. Changer is the most important here regardless, as all other of Missy's abilities stem from it. A shard behaves like a changer when a major component of the trigger event is either a body image issue or suffering caused by contrary societal expectations. In both cases, the parahuman in question finds themselves lacking, either in body or behaviour.
The prelude to the trigger event here, as it were, starts when Missy decides to act how her parents want her to with the intent of making them get along as a result. You know, be the bridge between them. The thing is, as she adapts to fit their expectations, she finds that she can't internally manage it. She dived in too deep, and is being both imprisoned and crushed by the pressure she is under. She has to bottle up her emotions to keep going, all the while wishing that she was different, that she could manage to fit the role she cursed herself with. Eventually, something sets her off - it doesn't have to be anything big, hell, it might fit better if it's something minor and insignificant. Regardless, she metaphorically loses her head, which leads her into encountering some form of physical danger that is likely to turn the metaphor literal.
... It doesn't fit perfectly - I wouldn't even say well - but it's the best I could come up with for the power offered. Maybe if I cleaned it up a little, removed some aspects and fine-tuned others, but I'm not going to do that. Now, as for my vote;
[X] Laetitia
I don't care for social thinker/master/stranger powers even a little bit, and as I just said, Ichthys is a little messy for me. Laetitia isn't perfect either, but I like her the most.