Which word best describes behavior that is whimsical and unpredictable?

The term "fickle" accurately captures behavior that is whimsical and unpredictable. When someone is referred to as fickle, it implies that their feelings, decisions, or actions can change suddenly and without warning, resembling a lack of consistency or stability. This characteristic aligns closely with the notion of whimsy, where individuals may exhibit playful or capricious behavior that does not follow a predictable pattern.

In contrast, fervor refers to intense and passionate feelings; fledgling indicates something that is new or inexperienced, often describing a young bird or a novice; and filibuster relates to a political term in which a person prolongs debate to delay action on a proposal. None of these words convey the sense of unpredictability associated with whimsical behavior as effectively as "fickle" does.