For the treachery of the Empire against the Noghri people. We were betrayed. We have been avenged.
~ Rukh before killing Thrawn.

Rukh is a recurring antagonist in the Star Wars franchise first appearing in the Legends timeline as part of the Thrawn Trilogy. During the trilogy, he was the personal agent and lieutenant of Grand Admiral Thrawn and was an agent of the Galactic Empire who aided Thrawn in eliminating his enemies and in reunifying the fractured empire. He first appeared in Heir to the Empire and served in a supporting antagonist roll throughout the rest of the trilogy until its end when he killed and betrayed Thrawn to avenge his people. 

Rukh later appeared as the secondary antagonist in Star Wars Rebels where he was made Canon once again. Just like his legends counterpart, Rukh is an agent of the Empire and is the personal assassin and lieutenant of Grand Admiral Thrawn who was first seen being dispatched to Lothal to aid in supressing the insurgency on the planet and aided Governor Arihnda Pryce in supressing the insurgency. 

Canon History

In 1 BBY, following the theft of a prototype TIE/D Defender Starfigther by the Spectres on Lothal, Grand Admiral Thrawn had Rukh dispatched to the planet to aid Governor Pryce in supressing the insurgency on the planet and to eliminate the rebels who were responsible for the theft. Pryce had suspected that the thieves would return to the crash site, so she arrived there along with an Imperial escort. Rukh arrived at the site as well and caught the scent of Garazeb Orrelious, one of the last living members of the Lasat species. He also caught the scents of Ezra Bridger and Jai Kell nearby who were trying to retrieve the power drive from the crashed starfighter. He approached the two of them and the rebels tried to masquerade themselves as Imperial soldiers, since they were wearing Scout trooper armor, and claimed that they had just finished patrolling the area. Rukh saw right through the trick and attacked the two of them knowing who they really were.