While oddly none of the main characters seem to have a catch phrase in Cwen's Quest it almost seems to be a requirement for the minor characters causing fans of the series to repeat them whenever a minor character show up.
One of Fred The Monkey's favorite tropes. Fred's, "Eee? Pah!", Sceb's "What the biscuit?" and, of course, Clydex's "I'm a platypus."
Othar Tryggvassen, Gentleman Adventurer! In addition to bellowing his name, Othar generally yells "Foul!" when a Spark defeats him in a manner that doesn't fit the rules of hero/villain combat that exist in his head.
Also, when somewhat-less-mad-than-regular scientists Agatha and Gilgamesh are thrust into the stereotype of their role, they tend towards the line "...and I can live with that!". (Things they've been able to live with thus far have included death rays, ranting at underlings like a cheap actor, and controlling an entire city and its army via a mix of mind control, symbiosis, and shared insanity.)
In Heavenly Nostrils, Marigold the Unicorn cheers "UNICORN!" whenever she does something awesome... which is fairly frequently.
Bert: "My world is a crotch!" Later, when he shows up again as a ghost (for a short time), it is confirmed to Torg that it's him when he says, "My afterlife is a crotch!"
The Unspeakable Vault (of Doom): The shoggies reaction to pretty much anything is "Oooh! Sooo coool!", but admittedly they are as dumb as you expect brainless pink goo to be.
Spoofed/subverted early on in Wigu, when the title character learns that he must be careful not to utter the catchphrase ("Spring into action!") of his cartoon hero Topato Potato, lest he be hunted down by the Catchphrase Killer.
Jason from The Word Weary answers "Who Cares?" off-handedly whenever someone asks him about another character's emotions.