Mavis is a privately-owned diesel shunter who works for the Ffarquhar Quarry Company, based at Ffarquhar Quarry.

When Mavis first came to Sodor, she was naïve, arrogant and did not take advice very well. She was mostly interested in her own ideas but was brought down to earth after her embarrassing incident at the Ffarquhar crossing. Still, she wanted to see more of the line and schemed to have the Troublesome Trucks push her past the level crossing beyond the quarry - this inadvertently resulted in a near accident for Toby. After rescuing him, Mavis resolved to change her ways for good. She is now more capable of keeping the freight cars and Troublesome Trucks in line and more willing to take advice.