Hugh Stamp was the right-hand man and Sean Ambrose's main henchman.
Hugh Stamp posed as pilot of the plane in which is Vladimir Nekhorvich and his boss Sean Ambrose. Stamp's mission is to cause the explosion of the device to kill Nekhrovich. Stamp succeeded.
Later, when Ambrose arrives at Nyah Nordoff-Hall. He was wary of Nyah. Ambrose, who was not at all suspicious, sliced a portion of the right atrial Stamp with a cigar cutter. Becasue of that, Stamp started wearing a band on top of the finger.
At Bare Island, he looks agent Ethan Hunt to kill him, but after a short confrontation, Stamp returns from Ambrose with Ethan that he captured. Sean kills pensemant but noticing the little finger on the corpse, he will realize that he had just killed Stamp.
Hunt had put a mask and Stamp had masked the true Stamp with Ethan mask to push Ambrose to kill him.