ML is the Hero who killed her in the first life. He was saved by her and when she disappeared he didn't want to accept her death. When the "Demon King" (Alyssa) appeared he could not recognize her because the demonic power changed her appearance, only when she spoke when she died recognized her from the voice.
The folks wanted to mutilate her body, burn it and do various things, but the hero said since he was the one who killed her, he should take care of the body and they accepted it. He made a coffin for her.
The hero begged God 100 days before FLs coffin, that he should be punished instead or let him share her punishment. God cannot bring back MC's soul because it was tainted by the demonic power. The soul needs to repent of the sins and let the darkness fade away, but Alyssa was so deeply crazy that she didn’t repent, so when God heard his prayer, God trusted ML with the duty of changing FL during the reincarnations. ML and FL reincarnating and remembering every previous life. They don't recognize each other. ML accepted it since he would fall in love with her anyway. He loved her in every life.
ML and FL have a daughter later on.
There are 3 couples in this manhwa, ML and FL being the first one. Then, the red-haired girl and the knight, and the blonde-haired girl and Austin (the black-haired mage)