Informal.
a typically pleasant, likable person who avoids causing trouble or dissension (sometimes used with Miss, Ms., or Mrs. instead of Mr. when referring to a female).
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(US) (or Brit Mr Nice Guy) noun [singular]
informal : a man who treats people kindly : a nice man
— used chiefly in the phrase no more Mr. Nice Guy
I'm tired of the way they treat me. From now on, no more Mr. Nice Guy! [=I'm not going to be nice to them from now on]
Useful english dictionary. 2012.