Which word indicates a person known for sudden and unpredictable changes of behavior?

The term "capricious" describes someone who exhibits sudden and unpredictable changes in behavior or mood. This word comes from the idea of a caprice, which refers to a whimsical or erratic notion or action. Individuals characterized by capriciousness may change their opinions, feelings, or decisions rapidly and without a clear reason, making their behavior difficult to anticipate. This aligns perfectly with the concept of unpredictability in behavior.

The other words do not convey this meaning. "Consistent" implies reliability and uniformity in actions or behavior, while "predictable" denotes a level of certainty in how someone will act. "Stable" suggests a lack of change or volatility, contrasting sharply with the notion of sudden changes. Therefore, capricious is the word that best fits the description of someone who shifts their demeanor unexpectedly.