Walt Warren

Walt Warren is a former inmate of Springfield Penitentiary.

History
Walt is generally a mild-mannered person, but was arrested for a minor offense and sentenced to a term in Springfield Penitentiary, where he had the misfortune of being cellmates with Sideshow Bob. Due to budget cutbacks, the city of Springfield announced that all minor offenders would be released from prison. Bob, hearing of this, noted the similarities in height and facial structure between himself and Walt and hatched a plan to escape and make another attempt to kill Bart Simpson. The night before the minor offenders' release, Bob knocked Walt out and performed facial transplant surgery, putting his own face and hair on Walt and vice versa. Then, the next morning when the minor offenders were set free, Bob simply walked out of prison in place of Walt (who was still unconscious from the surgery).

Walt, for his part, immediately realized what had happened and tried to warn the prison guards. Not believing the word of the prisoner they believed was Sideshow Bob, the guards put Walt in a straitjacket and locked him in a padded cell. Meanwhile, Bob, posing as Walt, had moved in next door to the Simpsons, but Bart was suspicious and insisted on making sure Bob was still in prison. Marge took Bart to visit the prison, where they saw Walt (looking like Sideshow Bob) in his straitjacket repeatedly writing "BART SIMPSON WILL DIE" on the walls with a pen clenched in his teeth. Walt was still trying to warn people of Bob's plot, but his writing on the wall was misinterpreted as Bob's usual behavior of threatening Bart.

Soon after that, Walt escaped from prison himself. He pretended to choke on his pen and then spat it into the eye of the guard who came to assist him. Still in his straitjacket, he grabbed the guard's taser in his teeth, stunned the guard, and made his escape. Walt then went to the Simpson home to warn them, but they still believed he was Bob, until Walt took off his shoes and revealed his normal-sized feet. Because Sideshow Bob had huge feet, the Simpsons finally believed Walt's story, and Walt explained everything that had happened. Then, searching Bob's house, they found clues indicating that Bob was planning to kill Bart at Five Corners, so they all went in pursuit of Bob, who had already kidnapped Bart.

Along the way, they stopped at a diner, where Bob had left a false trail (with a waitress who found "Walt" attractive) indicating he was headed for Mexico. The Simpsons believed the waitress and headed for Mexico, but Walt picked up on the waitress's attraction, didn't believe her, and continued on to Five Corners. He arrived in time to prevent Bob from killing Bart, and then managed to stall Bob until police from all five states arrived and the New Jersey cops arrested Bob.

Once all of the questions about his identity were resolved, Walt was a free man, having been officially released from prison. He moved away from the Simpsons' neighborhood and it's unknown where he ended up after that. It is also uncertain whether Walt remained stuck in Sideshow Bob's form or was granted plastic surgery restoring his true appearance, but the latter is more likely.

Trivia

 * Even though he's a minor offender, Walt managed to perfectly plan and execute his escape from prison, which could mean that either there's more criminal behavior in Walt's past than has been revealed thus far, or that while he and Sideshow Bob were cellmates, Bob bragged about his exploits enough for Walt to pick up a few pointers.