Try these options: "a taciturn personality", "a gentle personality", "a serene personality". All these words have similar meanings to the ones you mentioned but with slight nuances in the meaning.

I would go along with Richard with 'mild-mannered'. Also I would use 'easy going' to indicate the opposite of aggressive and extrovert. 'a quiet guy/girl' can be considered a positive quality -at least in Ireland it is.

1) You could say it, but it is not really clear what you mean. If you spoke without an accent natives would probably accept it. If you spoke with an accent people would question if you knew what you were talking about. Mild as a personality would not mean gentle and soft. You should use those words. Or maybe sweet. Mild would probably mean boring, featureless, plain. 2) Yes those can all work, but use commas instead of slashes. She has a calm, self-possessed personality. She is unflappable.