My Contract Partner Is the Infinite-Flow Big Boss

Synopsis: After completing the final mission in the Infinite Flow world, Yi San chose the reward of a chance to live in a new world. In this world, the gifted can have contractual partners, who can be any living being from another world. When Yi San arrived, he thought he could start a new life, but the contractual partner he summoned turned out to be his archenemy, the final boss of the Infinite Flow world.

Yi San: ... :)

Infinite Flow's final boss: ...

(The protagonist and the other characters are mostly friends, and there won't be a romance in the early stages.)