- fickle, volatile - marked by erratic changeableness in affections or attachments; "fickle friends"; "a flirt's volatile affections"
Antonym: constant (indirect, via inconstant)
- erratic, fickle, mercurial, quicksilver - liable to sudden unpredictable change; "erratic behavior"; "fickle weather"; "mercurial twists of temperament"; "a quicksilver character, cool and willful at one moment, utterly fragile the next"
Antonym: unchangeable (indirect, via changeable)