个性 (gèxìng) emphasizes the unique, individual qualities that make a person different from others — it's about 'individuality' or 'personality' as a distinctive set of traits. 性格 (xìnggé) is more about a person's character or temperament (e.g., outgoing, shy), while 人格 (réngé) refers to moral integrity or dignity. Use 个性 when you mean someone's distinctive style or identity.