In the novel, actually, they went back to the very beginning. Everything was the same, with only a few differences.
For example, everyone forgot what happened and didn't know who MC was. MC also forgot everything, and it was only Jeehee and the ball-like creature who remembered everything.
The ball-like creature was very important to the story because it was the one who explained the plot and the things they needed to do. That ball-like creature would be the one who would help MC to remember everything.
In the novel, Jeehee also stopped going into the mission for 2-4 months? (Couldn't remember). He would be very clingy to MC. That's why there was an intimate scene between them.
And remember the ring? The ring that MC wore and Jeehee could manipulate to weaken MC. They would exchange the role. Jeehee would be the slave, and MC would be the Master (not sure if he really took advantage of it).
I think the ending was like they had to sacrifice or give up their powers/abilities to come back to normal, something like that. And MC lived in that world, instead of "coming back" to his real world.

SOMEONE SPOIL THIS FOR ME PRETTY PLEASE