Rappa's Quirk is an unnamed strength-enhancing Quirk.
This Quirk appears to allow Rappa to increase the strength and the speed of his punches.
When he uses his Quirk, his fingers transform into a brass-knuckle like substance on the outer side of his knuckles. He is then able to throw a huge flurry of punches that are incredible quick, and immensely powerful. This Quirk is strong enough to shatter Eijiro's Red Riot Unbreakable form with ease, as well as whittle away at Fat Gum, a Pro Hero with a defensive Quirk.
However, this Quirk is unable to break Tengai's Quirk, despite its power.