Cassidy Wiliams, alias Angel Dynamite, is an inhabitant of Crystal Cove and a reccurring character in Scooby-Doo! Mystery Incorporated, as well as one of the original members of Mystery Incorporated. She first appears in "Beware the Beast from Below" as a radio DJ for Crystal Cove's K-Ghoul radio station. Before she is revealed to be Cassidy Williams, she is a friend of Scooby and the gang and is the only inhabitant of Crystal Cove who is supports them as mystery-solvers. After she is revealed, she is shown to be working with Mr. E, who has been hinted as, while she was still a member of Mystery Incorporated, once being her boyfriend. Upon his return to Crystal Cove, Mr. E also brought Cassidy along with him to help him seek out the Planispheric Disc and get revenge on Professor Pericles, as well as to help him keep an eye on the new Mystery Incorporated. Although she works for Mr. E, Cassidy is shown to have a great deal of concern and care for the gang. After she reveals her side of the reason why the original Mystery Incorporated left, the gang turn on her and no longer trust her as she had "lied to them from the very beginning."

When the Freak attacks Shaggy and Scooby at their home she and Ed Machine bring the gang to the radio station where she tries to comfort and give shelter to the gang only to have them lock her in her own booth. After attempting to contact Ed to let him know what has happened, she contacts Sheriff Bronson Stone and informs him of where the gang has gone and after rescuing her, she goes after them to make sure they are okay. During the first season of Mystery Incorporated she is seen with a 70's type outfit, with a huge afro, green bell-bottoms, green tanktop-ish type shirt, and platform shoes, as well as green eyeliner.

In the second season of Mystery Incorporated, Cassidy is no longer working for Mr. E and during the events of "The Night the Clown Cried II: Tears of Doom!", Mr. E seeks her out to ask her to come back and work for him but she refuses and states that she is no longer Angel Dynamite but just "plain Cassidy Williams." She also changes her look during the second season and cuts her afro to a much shorter curly hair and sports a trench coat and normal jeans and shoes. She begs to Brad and Judy, Fred's real parents, to stop hunting for the Conquistador treasure and to put Fred first but when they refuse she returns to K-Ghoul to start revealing the secrets of the Conquistador treasure and the curse of Crystal Cove. This has put her into real danger as Professor Pericles stated at the end of "The Gathering Gloom" that he and Mr. E must "silence her forever". She also begins spying on Brad and Judy. During the events of "Night Terrors", Cassidy is setting up the pictures of the Darrow family as a clue for Velma in her room to help her solve the mystery of the past mystery-solvers in Crystal Cove, including the original Mystery Incorporated, all of whom have vanished throughout the city's history. She is presumed to be dead after the events of the episode, "The Midnight Zone", and it is shown that the gang regret not reconciling with her before hand but she might be alive because in "The Horrible Herd" Proffesor Pericles said I will try anything to get the planispheric disc then he said including Cassidy Williams.

The younger version of herself is seen constantly throughout the first season of the series. Her younger self sported a more innocent look, with bow ties holding up her curly pig tales and sporting a yellow dress. She also wore glasses.