“One evil man decides to train Olivia. His name is Rex Dangervest. He was ruthless! He was dangerous. And he was very macho. He plots to defeat Bernard and kill Lovelace forever. He runs an invention-selling company named Gloomer-ator Incorporated. He plots to take over Brendam and make it gloomy FOREVER! (the word "forever" echoes) He wants to kill the Guardians.”
—Bernard describing Rex in the film's prolouge, a possible foreshadowing at his true role in the film.
Rex Dangervest is the hidden main antagonist of Brendam: Collision Course, a supporting character in Brendam: There and Back Again, and the main antagonist of Brendam: The Rise of Bernard, where he will apparently escape from prison. He is Professor Otto Wolfgang Zündapp's mentor, Olivia's former mentor, and Ben Livingston's partner in crime.
- Despite being the main antagonist of the movie, he has twenty minutes of screen time.
- He is absent in all of the trailers (with the exception of the final one) and appears in the wallpapers as a mentor to Olivia. But when the film is released, he is the primary threat to the Guardians of Brendam.
“Good evening, lemons. It is I, your boss, Sir William Rookwood. We will make the world gloomy forever. The citizens are fun-loving, peppy, and happy. They laughed at us, but now it's our turn to laugh back. Embrace your inner lemon. Let it drive you. This was meant to be alternate fuel's greatest moment. After today, everyone will race back to gasoline and we, the owners of the largest untapped oil reserve, will become the most powerful cars in the world! They'll come to us and they'll have no choice, because they will need us. And, they will finally respect us. So, hold your hoods high. After today, you will never be ashamed of who you are. Long live lemons!”
—Rex rising to power on the monitors, with a notably darker voice, while revealing his true colors.