from what I remember (its been a while) that woman (evil goddess) fought god himself and lost, and has been doing fuck all for a while now, until she noticed the MC and his party and started to look at them from afar. Out of the blue she just shows up and ask the MC to become her pet or something, then MC tells her to put some clothes on and she says only people who are stronger than her get to tell her what to do. She ask MC to a duel to establish that she is stronger than the MC since she is a being who has fought god, so MC is like fine but do it elsewhere and leave his girls out of harms way.
She takes the MC to a special dimension place she created that has huge thick walls made of special material (hard to break for normal people)
dont continue to read if you dont wanna get spoiler
they end up fighting and the MC is never overpowered, he never gets disintegrated by the evil goddess into nothingness then materializes himself, in fact they start to fight and the MC is not even fighting seriously (neither is she) then the MC slowly begins to break her psychologically by powering up over the fight and showing her that she cannot hurt him at all. One of his final attacks is so strong he ends up destroying part of the wall that not even she can break. At the end of the day she realizes she cannot touch him and she breaks down crying, the MC comes up to her and comforts her.... since she has been so lonely for who the fucks knows how many years maybe thousands he asks her that if she wants she can come with him and join his party.
Because she doesnt have a name she asks him to give her a name since she belongs to him now (she thinks he is her master cuz he showed him how strong he is by overpowering the fuck out of her during their duel) so he named her zero I think (cuz she has been in nothingness all this time, dont quote me on the name its been a while) anyways, she ends up falling in mad love for the MC... and
dont read this if you dont wanna see some mature spoiler
they end up having a discussion about his whereabouts then she asks for sex and mc gives it to her. She is the first girl in the harem to have sex with the MC in the novel