>>532003393
The guy is literally called "Hypnotist" or something. He starts appearing on the Yellow Line / 3rd line. If you're not able to see him, then it might be a bug. I was there just now to get an image of him, but for some reason no hypnotists showed up. Iirc they're kinda common there.

If you can see him, but never get hypnotized, it might be because you have defenses up as >>532003804 said.

For the final boss it seems if you have a certain amount of hypnosis on you, you'll tell him all your weak spots.