Answer
The answer is Yoko Ono. John Lennon's marriage to his first wife, Cynthia, ended due to his extramarital affair with Yoko Ono, a Japanese artist and musician. Ono became Lennon's creative partner and muse, influencing his music and activism. Their relationship had a profound impact on Lennon's life and career, marking a significant turning point in both his personal and professional journey.