This world is called the underworld because most of its inhabitants are monsters and demons, except for some people who defeat the monsters and are able to live with them

Alfred is one of those people who overcame monsters and found a way back to his world, but he kept going there and was known for his immorality and his extreme closeness to women, among other things

Elena was in her house conducting experiments to conjure ghosts, but she made a mistake and ended up in the underworld. She was wandering lost until she met the kindest person there, can help her. He was helping her search for a way out, and the news of her descent into that world spread like flames among the monsters

As for Alfred, he was looking for her to see if she was worthy of staying