Who is Clean Willy from The Great Train Robbery and what is their importance?

Clean Willy is a renown snakesman, someone who can slip into hard to reach places and unlock doors for their accomplices. When Agar tells Pierce that he needs Clean Willy to get into the railway office, Clean Willy is in Newgate Prison. Pierce gets word to Clean Willy that he needs him to escape, which Clean Willy does by scaling a rock wall. Pierce keeps Clean Willy in his own home to tend to the injuries he sustained scaling the wall. When it is safe for Willy to be on the streets again, he helps Agar get the keys in the railway office. Clean Willy then he turns on Pierce by informing the police. Pierce orders the snakeman's death.