Final answer:
Carl Rogers was the humanistic theorist known for his development of client-centered therapy and the use of unconditional positive regard to facilitate personal growth and self-understanding in clients.
Step-by-step explanation:
The humanistic theorist who believed in client-centered therapy and unconditional positive regard was Carl Rogers. Rogers's approach emphasized the patient's active participation in the healing process, contrasting with the psychoanalytic method that focused more on the therapist's interpretations. He posited that for therapy to be effective, a therapist must display unconditional positive regard, which means accepting the client entirely without judgment, along with genuineness and empathy. These principles are foundational in creating a therapeutic environment conducive to personal growth and self-discovery. Consistency between one's ideal self and real self was also a key concept in Rogers's view of personality development, with unconditional positive regard from parents contributing to healthy self-concepts in children. Rogers's influence continues to be significant in modern psychotherapeutic practices.