fickle player will have little love for your club(at least slightly higher than that of a mercenary) while still having very high ambition. He will therefore prioritize his career more than the progress of the club and should you slightly fail expectation, he will try to jump ship at the earliest opportunity.
basically high ambition + low loyalty. People have called it a bad personality but I’ve never found it to be an issue as long as you can be successful with them.