Mavis is a privately-owned quarry diesel shunter who works for the Ffarquhar Quarry Company at Ffarquhar Quarry.
Mavis is a feisty young diesel engine who is usually found working at the quarry. She is full of her own ideas and doesn't take kindly to advice, which can sometimes get her into trouble. A friendly diesel, Mavis is usually the first one Thomas turns to when the steamies and diesels need to work together. 
Mavis was particularly fond of putting the trucks at Anopha Quarry in different places, despite Toby's protests. Toby finally lost patience and left Mavis to her own devices. Mavis (urged by Daisy (Diesel in the television series)) started to ignore Toby's advice and ended up looking silly when she got stuck outside Ffarquhar.
Mavis was then forbidden to leave the quarry, but when a thaw increased production at Ffarquhar Quarry, Mavis persuaded the manager to let her go up to the first crossing in order to shunt trucks quicker, and formulated a plan to pass the level crossing with help from the trucks. However, the plan backfired when the trucks decided to bump Toby instead, and he ended up being suspended by flimsy rails over a rushing stream and Mavis had to help. Mavis confessed, but the Fat Controller and the Manager were pleased by Mavis' courage and honesty and allowed her occasional trips to Ffarquhar.
Mavis was arrogant and feisty until being brought down to earth after her embarrassing incident at the Ffarquhar crossing. Despite this, Mavis is a reliable, honest, hardworking engine, and shows respect for the steam engines.
Mavis is a based on the 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 sideplates so they could run near public roads. 20 of these locomotives are preserved (including 1 tramway example).
(BR Class 04)
Mavis is painted black with yellow hazards on her front and rear. She has her name printed in white on her cab and The Ffarquhar Quarry Co. Ltd. written in white on her sideplates respectively.
Mavis has had modifications throughout the model era. These include:
- Series 3:
- Her horn is BoCo's at a higher pitch.
- Series 4:
- Her middle lamp iron was removed.
- Her horn sound is actually Class 40's.
- Series 6:
- Her paint was given a matte finish.
- Her eyebrows became thinner and longer.
- Her middle lamp iron returned.
- Her horn sound is actually Derek's.
- Series 7:
- Her horn sound is actually Daisy's.
- Calling All Engines!:
- She receives a new horn sound.