Izuku saved Eri because she awakened something within him when he first laid his eyes on her. It was his fatherly instincts. Once he saved Eri and officially adopted her as his daughter they found the peace they so desperately desire. But it didn't last long. As an omnipotent being needed to have this multiverse fixed. So he decided to call on the help of Izuku and Eri. They agreed. What Izuku and Eri didn't know however is that this would be the best choice of their life