Does the character have to be a person, or can it be a monster? If it's a monster, then the most scariest monster in existence, and I mean it. The ultimately most terrible monster. It would be 'The Blob' from the 1980's move "The Blob"
The blob has an acid touch, and it's super sticky. If you get it on you, you're screwed. It can pull you inside of it, and dissolve you, and every it eats more flesh, it gets bigger.
The blob can't be killed. You can only freeze it, and it has to stay frozen. You can't let it thaw out. When the blob freezes, it turns into a bunch of purple diamonds. You have to lock them away in an arctic region.
In RPG world, the only other way I could say to get rid of this thing is to cast X-Zone on it. That sends it into another dimension.
Some people don't like me and find me annoying. I'm sorry you feel that way, and I'm even more sorry that I don't. care