And in the end it ended up bing his last spirit detective to undo everything that took centuries to prevent.

So the whole time while king Yama was trying to prevent the a and s class demons from entering the living world, and koenma sent yusuke to stop a power hungry mad man with a gambling problem, and an ex spirit detective with multiple personality disorder

The crazy part about this is that after yusuke went to demon world to train, he was the one to inadvertently open up the barrier between demon and living world by hosting a tournament to see who would be the king of demon world.