hi I was told about someone called prism by a guard in the nashkell mines. Is he a usable character? and if so, is he any good? and finally, WHERE THE HECK IS HE?????

Prism has stolen some gems to use with a sculpture and therefor is he sought by the law.
I don't remember where he is. Maybe in the south part of the Nashkel mine site.
He's not able to join your group, but you can save him from the bountyhunter Silverwolf (or something like that).
Silverwolf could be hard to kill, but he has some good stuff so it's worth it.

Prism can be found around there (Nashkel Mines). He will be near a large sculpture in the side of a hill, that he is currently working on. The guy trying to kill him is Greywolf - a fairly nasty character, and it is up to you whether you want to cooperate with him or not. I would fight him if I were you, as he has a very nice item on him... although I won't spoil it with any more details.

Just a thought... being the good guy gets you more XP and reputation: a) Greywolf + item, b) the XP Prism gives you.
You may still be a bastard later on and hand the stones Prism stole over to the fat guy in Nashkell.