Mavis is a privately-owned diesel shunter who works for the Ffarquhar Quarry Company, based at Ffarquhar Quarry! she also works at the Sodor Slate 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 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 trucks in line and more willing to take advice.
Unlike many Diesels, Mavis shows respect for the steam engines and unlike most engines, hauling trucks makes her feel important. She is a reliable, honest, and hard-working engine.
Over the course of time, Mavis has matured and acts as a motherly figure to the other engines. As a result, she keeps a stern attitude with the troublesome trucks and holds zero patience towards troublesome antics like those from Diesel. Although Mavis means well, she can at times feel cooped up in the quarry, but always cheers up at the chance to stretch her wheels
Mavis is a based on a BR Class 04 0-6-0 diesel locomotive. She is more specifically based on the BR Class 04's that worked on the Wisbech and Upwell Tramway, as they were fitted with additional cowcatchers and side plates so they could run near public roads. Twenty of these locomotives are preserved, including one "tramway" example.