Self Personality Control

Take somebody who had the power to literally reach inside their own minds and truly rewire their personality. Not fake it, not act convincingly, but actually manipulate their own psyche at a whim.

Now, if you were writing this person, any setting, any background, how would you write them? How useful/dangerous could and would you make their ability?

If there's a way to 'reset' back to the original personality it'd make them the ultimate deep-cover operative.

Different kinds of personalities suit different jobs/tasks: for example, I would do well at any kind of public speaking, but my brother beats me whenever hard work is involved. This is because he's a more diligent and focused person, whereas I'm more flighty, but keep my head in front of crowds. A person who could manipulate their personality like that could adapt themselves to any kind of job.

"Oh, I need to intimidate someone? I'll change myself into a blunt scary person. Wait, here's a cute little kid....I'll change myself into a cool guy so he'll do what I say." Though that kind of character seems difficult to write about.