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 and Diesel, started to ignore Toby's advice and ended up looking silly when she stuck outside Ffarquhar.
Mavis was then forbidden to leave the quarry, but when a thaw increased production at Anopha 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.
In 1987, Mavis was involved in a collision with a lorry and Toby had to do her work while she was absent.
Mavis was arrogant 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.
She is kind to Thomas and Flora. And has a love interest in Toby. Her best friends are Daisy and Rosie.
Mavis is based on a BR Class 04 0-6-0, fitted with additional cowcatchers and side plates so she can run near public roads, designed and built at Drewry Car Co. in 1952, approx. 26 ft 1/2 in long.
Mavis is painted black with hazard stripes on her front and rear. She has her name "Mavis" and number "No. 2400", "No. 145", "11103", "D2203", later "FQC 1" written on each side of the outside of her cab in white and "The Ffarquhar Quarry Co. Ltd." written on both sides of her plates in white.