Who is Mad from The Last Continent: A Discworld Novel and what is their importance?

Mad is a psychotic dwarf who rides around the outback of the last continent, Fourecks, in a heavily-armored cart pulled by a team of armored horses, Mad gives protagonist Rincewind the Wizzard a ride to civilization in Dijabringabeeralong. When Mad mentions a coastal city, Bugarup, Rincewind sets his mind on booking passage thence for home. As a gang of marauders chase them, Mad orders Rincewind to shoot at them with a crossbow while Mad supercharges his horses.