The psychologist known for their theories of personality and the unconscious mind is Sigmund Freud. Freud's pioneering work established the framework for understanding the human psyche, including the motivations behind behavior and the influence of the subconscious. His concepts of the id, ego, and superego provided insight into personality structure, while his theories on defense mechanisms and dreams expanded the field of psychology. Freud's impact on both psychology and culture has been profound, making him a central figure in the study of human behavior.