Dorian Wormwood is a retired stage actor, famous for playing Sherlock Holmes.
He is distinguished-looking, with gray hair and mustache (the mustache is false, because he must play other parts with no facial hair).
He says he wishes he could solve real mysteries like the gang, instead of just pretending on stage.
Scooby-Doo, Where Are You? (DC Comics)
He wished to return to the stage and wanted to demonstrate he could still act. Because of this, he played every person that the gang met in this case, doing rapid costume and makeup changes in-between. He also staged his murder, falling over a cliff after being "shot" and into a waterfall (most likely into a concealed net). Daphne Blake caught on first, because the name of every character he played was associated with Sherlock Holmes (e.g. Moriarty, Hudson, and Riechenbach).