Walking the line between genius and insanity.
Jasper Thickneck is a plot-device NPC. If your players are frustrated with a lack of perceived progress, or if their running in an unexpected direction is beginning to exceed your capacity to tie it all back in to what you had planned all along, he can pop up to dispense bizarre and/or cryptic advice and nudge them toward a given path. Do make sure the PCs learn about his mental state either before or shortly after they meet him. Half the fun is making them guess whether his ramblings are actual advice or just another weird dream.
As a plot-device NPC, Jasper is exactly as powerful as he needs to be in order to be found wherever he is encountered (and it’d be totally reasonable to encounter him on the road, in the wilderness, or on one of the more popular planes). If your players are the kind that attack innocent-bystander NPCs for their own amusement, FemmeLegion recommends statting him out to be more than a match for the entire party put together, because she really hates players like that.
Here’s a potential image of Jasper. Of course, Jasper can look like whatever you want him to look like, to suit the plans you have for him. This is the image that Duskreign likes, because he looks serious enough that his advice, odd as it is, seems like it should probably be taken seriously. Also, as FemmeLegion suggested, the guy is as tough as he needs to be, and should be able to hold is own (at least) against a bunch of unruly, npc-murdering PCs.
Once upon a time, Jasper Thickneck was a renowned soldier for his clan. Such was his devotion to his people that he turned down repeated offers for the adventuring / mercenary life, saying that he was born to defend his home.
Then came the demon Vug’Nyagar, who threatened all of the world. Jasper traveled five hundred leagues from his home to join a band pledged to stopping the demon’s terror. He and his comrades won the day, but Jasper literally lost his mind to the demon’s ravaging. While he could understand when others spoke to him, he had barely any vocabulary left of his own, and not a single memory.
Merkin Silverpate, a cleric of the goddess of prophecy, tried to undo some of the damage, but alas – his Lady is forever looking forward, never backward. Jasper is now functional enough to live independently, but he pretty much lives entirely on intuition at this point, and he makes other dwarves nervous because he often comes off as being whimsical rather than sensible. While other, more reasonable dwarves would amass an army to defend against intrusions from the Underdark, Jasper goes toddling off to the nearby human settlement on some completely unrelated task – except sometimes it ends up being perfectly related and the best thing he could have done.
Jasper is also frequently plagued with odd dreams and visions, and is forever sharing them with whomever he thinks would benefit from hearing them. Only about one dream/vision out of three is actually meaningful, but that’s good enough for the people who visit his home or encounter him wandering. It’s better than they’d get if they didn’t see him at all.