Flash was a young Caucasian male with curly, red hair.
Little is known about Flash Flannigan, but he was always shown wearing a scowl.
Not much is known about him, apart from the fact that one day for unknown reasons, he stormed off stage of the Mystery Kids and quit, never to be seen again. He was since presumed dead (killed in a car crash, according to Susan), and the Mystery Kids themselves retired.
What's New, Scooby-Doo?
The Mystery Machine (now owned by the gang) was thought to be haunted by his ghost, but it was actually Susan Dinwiddie manipulating it by remote control, who was trying to make a big publicity stunt for the Mystery Kids' comeback. Andy and Mandy visited Flash's grave before the comeback since, despite what happened between him and their mother, they still idolized him.
- Flash's grave seems to have been made simply because everybody thinks that because he vanished, he must be dead; the circumstances of his death, if it even actually happened, weren't revealed. A vigil would've been more appropriate, but that wouldn't have helped the rest of the episode.
- It's currently unknown why Flash stormed off and quit the Mystery Kids. However, because of Susan Dinwiddie's obsession of making her kids famous, it's possible that he quit in protest of her obsession, as he felt he didn't get the credit or respect he deserved.