The other six fermions are called leptons, a name derived from the Greek word λεπτός (leptos) meaning thin, delicate, lightweight, or small.
These particles don’t need to bind to each other, which keeps them „thin” in a certain sense. Originally leptons were considered the „light” particles and hadrons the „heavy” particles, but the discovery of the tau lepton in 1975 broke that rule. The tau (the heaviest lepton) is almost twice as massive as a proton (the lightest hadron).