This is what I remember from the manga. It starts off with the main character getting transported to another world, however if I remember correctly, he wasn't the one that was supposed to be summoned, or so thought the people who summoned him.
The story progresses and the MC becomes super powerful, and the demon king starts to come to his house and tries to makes friends with him because he knows how powerful the MC is. The MC also runs into a a white demon which he defeats, and it turns into the form of a human woman and starts to follow him around, and they are pretty much a couple.
I also remember that there was a super powerful demon locked up in the basement of the castle, if I remember correctly, and it gets released and no one can stop it. However, it harms his girlfriend/wife, and he pretty much goes berserk and attacks the super powerful demon and wins. After that, the demon starts to follow him around and I believe calls him master.