Storywise I'm thinking my character, ironically named Prosper, tells those who'll listen he's the son of a rich merchant family from a far off land who's ship left port while he was passed out drunk in a brothel. In reality, his father was a sailor who's ship went missing at sea, and his mother was a prostitute who was murdered in front of him by a drunk, wealthy merchant from a far off land. He survives as a wharf rat scavenger, pulling in debris and bodies at the docks with a hooked driftwood staff. One day, he pulls in the body of a soldier of unidentifiable origin. When he touches the badly rotted insignia affixed to the soldier's chest, a column of light breaks through the clouds basking Prosper in a divine glow. He is momentarily awashed in visions of war in bloodshed. An unearthly booming voice explains he has been given power so that he may make amends for a grave and monumental sin. The only further information he is given is that he must persevere through many trials and travails. He emerges from this experience a changed man. He sets out to help as many people as he can, and reveal to sinners their transgressions so they might repent. He is faced with two major problems. The first, is that no one else witnessed the divine event that granted him his power. Knowing no one will believe him, he is wary about revealing his powers for fear of being lynched. Second, is that Prosper is a bit touched. It hasn't occurred to him that people might accept his help more readily if he shaved, or bathed, or stopped wearing his trusty, but filthy, cloak. The rusted chainmail from the dead soldier he wears and the somewhat menacing driftwood staff he still carries with him everywhere don't help, either.
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A change to the back story might be in order, then. How about this: As he was pulling the soldier in, a freak surge in the tide dragged him into the water. As he started to drown, a beam of light guided him to the surface, where he emerged in an impossibly huge pool of blood in the middle of an endless battlefield with a red sky and black sun overhead. He hears the voice of the Guilty God, and is dragged back under. When he resurfaces, he finds himself freezing his ass off hanging onto a large chunk of flotsam. The ship just happens to find a thirsty, hungry, and cold Prosper yelling for help a suitably uncomfortable period of time later. Maybe if Prosper runs into someone that recognizes him later, it turns out he went missing months, or even years before.
I'm thinking Prosper is 21 or so, but he looks significantly older. He's likely never shaved his beard, and his hair hasn't been cut in years. His nose has been broken a few times, and a few of his already horrible teeth have been knocked out. His divine experience out at sea could have had the strange side effect of salting his beard and hair with some grey as well.