Paranoid, nervous, and usually careful, Prosper was a smuggler, ferrying several large cargos of ingots from the steelyard of The Pitt to outlying areas. His operation was soon compromised and his crew mostly butchered. The survivors were sold to slavers heading toward Paradise Falls.
Into the Pitt: You can buy him and the other slaves in the slave pen from Ramsey or you can kill all the slavers to get the key to open the pen yourself. After either option you can then talk to Prosper and take his clothes from him or take the clothes from the dead slave in the corner.
If you follow him, he and the two other slaves will try to head to the entrance of Vault 101. They will likely be killed if not escorted, although they can survive the journey at which point they will "live" at the entrance of Vault 101.
He will not equip any weapons or clothes given to him via reverse-pickpocketing, meaning you can't improve his chances of survival by doing so.