Angel's Master is the name given to the man who trained Angel, for obvious reasons. He was a human member of The Organisation who fought with twin swords. At some point in the past (before Angel met the rest of the party), he was killed while on a mission with her by a deathfin from Blood Mountain.
Although some viewers have nicknamed the (unnamed) Master character "Rough", word-of-god is that "(Rough)" appears in brackets in the title because Demonac wasn't satisfied with some of the animated effects, and wanted to make a new "final" version of the episode once he had time to fix that and create appropriate Monitor music (to replace the Vampire King Theme used for the action in the latter half of the video). Whether this final version will ever actually happen is a different question...