The protagonist get the ability to confine people inside. He is supported by a devil and her subordinate. He learns trick and such from the devil to brainwash others. The more he enslaves people, the more his ability level up. His ability is actually shared with the devil and the devil use it to wage wars in the demon world, it happens way later so you can ignore this part.
The protagonist have a bottom line : as long as people don't mess with him or someone close to him. At some point he will capture some people but release them 'unharmed'.
He didn't brainwash all the female leads, some are willingly follow him. For examples : he enjoy the feeling of having a non-brainwashed gf or having a non blood-related little sister-like existence.
He slowly get some financial power in the real world.
Only things i didn't liked is how author don't just finish minor antagonists, sometimes the events looks a bit dragged... also the fact that sometime author seems conveniently forget some of protagonist's abilities.