Merp is a defective Irken (Designed for Invader ZIM: The DOOM Movie!) whose defect (apart from her missing legs) is an unnatural fixation on revolution and uprising. For obvious reasons, the Irken Empire would not tolerate such a defect, and so she was processed and sent to Defective Area 9, to be a snack slave for the rest of her life. While there, she comes up with plan after unsuccessful plan to escape.
However, one day, she hears word of a defective who managed to avoid being sent to Defective Area 9: Zim. After the defectives witness footage of his interruption of The Great Assigning, Merp notices that Smoots has become horrifically in love with Zim, idolizing him and ceaselessly daydreaming about him. Seeing this as an opportunity, Merp helps Smoots escape, in hopes that she will bring him back to lead a revolution.
This escape is what kicks off the plot of Invader ZIM: The DOOM Movie!
The symbol on her forehead is a "Defective" symbol that Stanley Orr came up with when designing her character.