Resolve and Willpower

Yep, Rain ended up bonding with his Esper daughter, though this one was through a contract, not fusion. This happened during the events of the Esper of Ruin, when she awakened her powers, and saved Rain, requesting he make a contract with her in order to defeat Chaos Bismarck. While bonded to Sophie, Rain's body constantly heals at an accelerated rate, to the point that Bismarck's destruction of their world didn't leave any lasting damage on Rain's body. This also restored Rain's lost eye, leg, and arm, allowing him to fight without any restraints. While bonded to Sophie, Rain becomes much stronger, to the point Bismarck thought Rain had become a god of sorts.