This isn't pathfinder-specific but I think it can be easily modified to fit into any relatively magical game.
A desperately poor, lowest class homeless man eats a lot of poisonous mushrooms in a moment of starving desperation, and goes on a trip from which he never really comes back. Latent potential for sorcery begins to manifest itself as he sleeps fitfully in a gutter, but he doesn't recognize what it is or how he's doing it. He hears voices all the time, thinks they're semi-benevolent spirits talking to him, ordaining him as their champion, empowering and tasking him to uplift the weak, and mete out bloody justice to the well-off who step over (and on) the unfortunate.
In his ignorance, his magical practice is infused with mysticism. He will divine answers by casting discarded chicken bones over a scribbled nonsense diagram or particularly interesting stain in the road, examining them carefully (and actually casting nothing), making a wild guess and assuming the idea is divine revelation. He carries bogus talismans and charms, and depends on them to help fight or ward off evil. He goes far out of his way to avoid seemingly random items, creatures and places, considering them taboo. He speaks to the spirits of all living things, from trees to stray cats to innocent bystanders who have no idea what he's talking about. He takes his hallucinations and delusions as absolute truth, and views everything else with doubt and suspicion. He indulges in drink, drugs and mild poisons whenever he gets the chance, because they help him hear the spirits.
He is a demented, drug-addled, witchdoctor hobo, and a budding reality warper. He thinks of himself as a spiritual superhero, but he's more like a wandering nexus of chaos. He just sees the world wrong, and it bends to accommodate him. His spell list is filled mainly with illusions, transmutations, and conjurations, to facilitate and reinforce his distorted view of the way things are.
I had that guy as a PC once, and he was both entertaining and baffling. Best taken in small doses, but his erratic behavior makes it very easy to insert him into any situation, or have him wander off abruptly if his presence is getting old. He works best if you keep the mechanics of what he's doing secret, so it's unclear if he's, for example, casting speak with animals and using Diplomacy against a horse, making an untrained Handle Animal check on it, or just babbling at it. As DM you might consider giving him a permanent mind blank effect, or the Madness ability of the allip monster, to represent and mechanically support his mind being so far away you can't even reach it magically.
