Motormouth (Harley Davis) is a fictional character appearing in American comic books published by Marvel Comics. The character was created by Paul Neary and developed by writer Graham Marks, with initial designs by Gary Frank. As the title character of her own series, Motormouth (later Motormouth & Killpower) was part of a line of original comics released in the early-to-mid 1990s by Marvel UK. She is a teenage street rat from London, England, with a fierce temper who curses constantly. She later gains the ability to emit a sonic scream and jump across realities using technology.

Motormouth ran for 12 issues, with Harley and later her partner Killpower going to various alternative realities from the regular Marvel Universe.

Harley Davis was orphaned at age 11 and grew up as part of a gang in the East End of London. They survive by foraging and stealing. Her incredibly foul language earns her the nickname "Motormouth." When she is 17, Harley is discovered by Laarson, an agent of Mys-Tech. Harley is a candidate for his MOPED (Mind-Operated Personal Dematerialization) technology, which allows people to jump between alternative realities, but it will only operate on people with certain strands of DNA. The unit is disguised as a pair of training shoes, which Harley finds and tries on. At the same time, Laarson is being executed by Mys-Tech's techno-wizards, and agents are sent to find the MOPED units. To save her and the units, Laarson's subordinates activate the technology and send her to another universe.

While on the run from Mys-Tech, Harley is sent to a future in which the world is covered in shopping malls. She purchases a Soni-Muta 500 Unit, a microchip that allows her to mentally tune in to any radio station she wants. Harley returns to her own universe and is captured and put to work by Mys-Tech. She escapes after a mission to Tokyo.

Harley makes a random jump to escape and ends up on the planet Tekron. Killpower, an agent of Mys-Tech, is sent to find her. Unable to escape him, Harley reasons with the man instead. Killpower is intrigued by Harley and joins her as her partner. While on Tekron, Harley is injured when a stray bullet hits her in the throat. Killpower uses her Soni-Muta 500 and MOPED technology to cure her, giving her a sonic scream and the ability to travel between universes without the aid of technology. Both are then forced to flee from Mys-Tech's influence.

Motormouth and Killpower then appear in the Battletide mini-series. Death's Head is hired by Mys-Tech to retrieve Killpower. While the two fight, Motormouth and Tuck (Death's Head's partner) go on a shopping spree on an alien planet. Motormouth and Killpower later plan an attack on Tyburn, a powerful Mys-Tech agent. Harley is able to steal their MOPED unit and neutralize the tracking devices Mys-Tech has implanted in the duo. Tyburn survives the attack.

Motormouth becomes an essential part of the Mys-Tech Wars mini-series. At first, she is just one of many superheroes attacking the main Mys-Tech complex, hoping to take down the Techno-Wizards, the heads of Mys-Tech.

The wizards fight back successfully, slaughtering most of the heroes. Death-Head kills them in return. Unfortunately, this endangers all of Earth. A fortunate fall from Killpower allows six heroes, Motormouth included, to break free of the melee and patch themselves into the required equipment so they can save the Earth from destruction. The others, such as Doctor Strange and Professor X, bring powerful forces to the mix. Motormouth ends up simply sitting in the sixth chair to fill out the required number of entities.