Backseat driver.
The Grunt Rider is an infantry unit which replaces Suicide Grunts. The Grunts chosen to lord it over captured Brutes still lack much common sense, so going from carrying a backpack to being a backpack may not be a step up. Grunt Riders have a simple partnership with their Brutes: the Brutes punch things with their fists, while the Grunts tell them where to go and throw plasma grenades at enemies, allies, small animals, inanimate objects, and strange noises. The Brute's attacks as-is, they can upgrade their punch force with Grunt-engineered plasma gloves by investing in Punch Harder, You!
Unlike methane and infusions, the Grunt and Brute pairing is not a perfect match. Among other quirks, some Brute Riders have a tendency to develop an obsession with biofuels, power management, and gladiatorial fights, much to the annoyance of everyone else in the rebellion. Other Brute Riders suffer "accidents" at the fists of the Brutes if they get too bossy. The rest simply hope to survive long enough to get promoted to Locust driver or methane hauler.