So the general gist of this quirk would be that the user is able to manifest different physical forms based off how they view/picture the different mental states, emotions, and moods in their mind. The strength and power sets of said manifestations are constantly changing and altering as well as fluctuating in power based off the users mental health, the situation they're in and how they're processing it mentally, as well as other minute factors.

Downsides are obvious, it's a very random and hard to train quirk. One would need to work on absolute mastery over their mind to utilize this quirk to it's fullest. However even with absolute mastery they'd still be vulnerable to other factors in their day to day lives as well as on the job. A friend dying in combat or a civilian being tortured could act as a power boost if the Quirk user manifested their personification of anger or it could weaken them if they were using a more positive manifestation.

Basically the quirk and it's many uses would boil down to how the user sees the different parts of themselves. They could see their anger as this writhing mass of fire and smoke that can never be put out or they could imagine their anger as something cold and distant that encases everything it touches in frost.

But those are just my general thoughts, mostly came about after I started doing an exercise a friend of mine taught me for when you're feeling down or depressed. Draw what you thing your emotions, moods, feelings, etc look like. Thought it'd be a neat quirk idea if one could become what they imagine.