Greef Karga is the Bounty Hunters' Guild expeditor on Nevarro, played by Carl Weathers in The Mandalorian from 2019. StarWars.com defines the job precisely — he "ran the trade on Nevarro, a middle-man, a connector between clients and bounty hunters" — and notes that by the second season he is "Magistrate of Nevarro", cleaning up the world he used to profit from.
Infrastructure, personified
His real contribution to the setting is administrative. Through Karga the series explains how bounty hunting actually works: tracking fobs issued from a central pool, pucks with the quarry's face and price, guild protocol about who may take what, and the fact that the client's identity is nobody's business. He is the reason the profession reads as a trade rather than a genre convention.
Adversary, then ally
He turns on Din Djarin the moment the Guild does, hunts him, is nearly killed, is saved by the child, and switches sides — a sequence the show plays as pure self-interest right up until it isn't. Weathers plays him as a man whose warmth is entirely genuine and entirely for sale, which is a difficult combination to sustain.
Governor
By the third season Nevarro is a functioning trade port with schools and a droid workforce, and Karga is running it. The arc is one of the few places the New Republic era shows what reconstruction looks like from below: not a fleet arriving, but a fixer deciding that legitimate business pays better.