The meanings of inconstant and fickle largely overlap; however, inconstant implies an incapacity for steadiness and an inherent tendency to change.

// an inconstant friend

Some common synonyms of fickle are capricious, inconstant, mercurial, and unstable. While all these words mean "lacking firmness or steadiness (as in purpose or devotion)," fickle suggests unreliability because of perverse changeability and incapacity for steadfastness.

// performers discover how fickle fans can be

The synonyms unstable and fickle are sometimes interchangeable, but unstable implies an incapacity for remaining in a fixed position or steady course and applies especially to a lack of emotional balance.

// too unstable to hold a job