Walter Peabody was Alan Dinsdale's assistant at the Hambridge University Library. However he was secretly involved in Travis Sherman's plan and under his orders to steal the Tome of Doom, and replace it with a fake.
Having done so, he ripped out several pages, sending some to Robert Zabrinski, one to Mindi Stiles and two to Travis Sherman. Keeping the remainder he used it to summon several ghosts and shadow monsters to terrorize the Library.
Everyone from the library had left from the mysterious hauntings, except him. After Shaggy Rogers and Scooby-Doo, along with the rest of the gang, go on a wild goose chase, they met up with a spooky hooded figure. After they capture the figure, it was revealed to be none other than Walter. The gang found a telegram for Walter, with instructions for the Tome of Doom reading, "S.D.", Walter was then hauled off to jail, with the telegram remaining ambiguous, till the final episode.
Walter's costume was similar to the Phantom of haunted isle, another Socoby-Doo villain