Like all members of the Pixie Dusters, Mustardseed is shown to be aggressive to anyone who they encounter. Despite her diminutive size, she and the other fairies are able to ride their motorcycles perfectly.
Role in the film
Mustardseed and some other members of the Pixie Dusters are first seen parking their motorcycles in the Swamp Gas area where Ian, Barley, and their father witness them assaulting an innocent elf, causing him to drop his soda. As the elves enter, Mustardseed and the other Pixie Dusters swarm the area while Ian purchases a refill of his gas, followed by some Cheese Puffs and a bathroom key added, due to Barley's impatience. Fortunately, the three manage to escape from the Pixie Dusters to avoid getting into a brawl.
When Barley argues with Dewdrop about the Pixie Dusters' inability to fly, Ian and Barley's father topples over the Pixie Dusters' motorcycles, causing Mustardseed and the other members to rampage toward the elves. Just as Ian and Barley escape via Guinevere, Mustardseed and the other members chase them across the freeway via motorcycles. Throughout the chase, Ian driving on Guinevere manages to lose them while being instructed by Barley due to his diminutive size, causing the elf brothers and their father to get away from them.
Later when Laurel and Corey go to rescue Ian and Barley, Mustardseed and the other Pixie Dusters witness one member crashing onto Laurel's windshield, just as the Pixie Dusters arrive towards the injured member as they retreat to heal the injured sprite. Mustardseed's further fate is unknown.