In The Quest for the Holy Quail, Geoffrey instructed Merle to kill Queen Griddle when King Allfire wouldn't read the ransom note, and so she tried to feed her to Yugmort the Many-Fanged. When this failed, Merle teleported back to Castle Threadbare with Geoffrey and the Evil Knights.
In Knights & Knightresses, Merle seemed to take the side of an army of barbarians rather than Geoffrey (whom they were going to eat), giving them land and planning to dominate the world. However, according to Princess Flame, the flaw in her plan was that the world is round and not flat.
In Merle's Mirror, Merle hypnotizes Griddle with a magic mirror and makes her try to kill Allfire. When Flame smashed the mirror, its effects were reversed to Merle. Flicker made her turn Geoffrey and his Evil Knights into beetles, and Flame made her turn into a frog, and she chased Geoffrey and the Evil Knights away, intending to eat them.
In Sir Hare, Merle was assigned to create a giant knight for fighting the dragon knights, however, the knight turned out to be a giant rabbit.
In A Killer Makeover, Merle transformed into the Black Knight, and Marta of the House of Stewart. As the Black Knight, she gave Count Geoffrey a makeover and taught him to be more evil, while as Marta, she manipulated the dragons into giving up their weapons, until Flame discovered her true identity.
In Shamrocks & Shenanigans, Merle for once did not trick the dragons, giving them a four leaf clover in exchange for the gold they found.
In King for a Day, Merle disguised herself as a "drop-dead gorgeous damsel" to distract King Allfire so that Count Geoffrey, whom she turned into a clone of Allfire, could infiltrate Camelhot and steal the Golden Gauntlet of Gallopingosko. Eventually Merle got fed up and left for Vegas with the Evil Knights.
In The Isle of Dwight, Merle said that Griddle's first husband, Sir Herman the Nearsighted, was not of noble descent, on the basis that his shield has no record, and that by one of Allfire's own laws, if the shield was not produced in 48 hours, Allfire and Griddle would no longer be king and queen and the throne of Camelhot would go to Count Geoffrey, the nearest nobleman. However, Blaze found the shield within the 48 hours, foiling Merle's plan.
Merle's final appearance was in Geoffrey's Evil Pancakes, in which the Evil Knights catapult her at Sir Loungelot. She lands in Loungelot's arms, and he freaks out and runs off, dropping her to the ground. Strangely enough, Merle does not move or speak at all in this role, which (along with her non-reappearance in later episodes) hints that she may be dead, of unknown causes.