-Spoiler Alert-

The Creeper was the costume of Mr. Carswell, a bank manager.

Physical appearance
The Creeper was a green-skinned phantom/zombie, with dark red hair and yellow sclera. He wore a dark green trench coat, with dark green pants and black shoes.

Personality
He had a violent temper, as shown when he brutally beat up a bank guard. He also didn't appear to be very smart as he fell for Scooby-Doo's incredibly obvious hen disguise. He was obsessed with the paper that the bank guard gave Mystery Inc (because it was the only evidence that Carswell was secretly robbing the bank).

Powers and abilities
He could supposedly walk through walls, allowing him to rob the bank. In fact, the trick worked because Carswell was the bank president and had complete access. He also had immense strength as he knocked the bank guard out with ease and ransacked his entire car without damaging it on the outside.