To be fickle (inconstante, veleidoso, voluble) is to be inconsistent, unreliable or capricious (caprichoso). The noun is fickleness.
Examine these examples:
• Mr. Podemos is a fickle (voluble) person: he is not reliable.
• Some consumers are fickle (inconstantes).
• The fashion business is very fickle.
• The fickleness of some politicians nettles me. Click here to review the word “nettle”.