fickle
Meaning: shifting in character, inconstant
Usage: In Greek dramas, the fickle gods help Achilles one day, and then harm him the next.
mercurial
Meaning: characterized by rapid change or temperamentality
Usage: Though he was widely respected for his mathematical proofs, the mercurial genius was impossible to live with.
whimsical
Meaning: fanciful, full of whims
Usage: The whimsical little girl liked to pretend that she was an elvin princess.
precocious
Meaning: advanced, developing ahead of time
Usage: Derek was so academically precocious that by the time he was 10 years old, he was already in the ninth grade.
capricious
Meaning: subject to whim, fickle
Usage: The young girl’s capricious tendencies made it difficult for her to focus on achieving her goals.