Dermot "Leon" Cusack was a Human male, who served as a yeoman in Starfleet in the 23rd century, attached to Captain Christopher Pike . He had a reputation as something of a rogue.
Yeoman Cusack transferred with Pike from Pike's prior command, and was the only one to do so. When Pike was going over the personnel files deciding on his final crew for the mission, Cusack was there with a bottle of Irish whiskey to help him.
After an attacking force was driven off from Starbase 13, Pike took Spock, Cusack, and Carlotti to the station to help with the situation. Cusack and Carlotti set up a medical triage station to help with the injured. The Enterprise then went to check out a loss of communications with the Pharos Project, and Cusack went with Number One's landing party. They were successful in their mission and returned to the ship safely. The Enterprise visited Rigel VII soon after and took some shore leave. Cusack continued to show his worth to Pike by assisting him with all of his regular tasks promptly. Cusack spent the evening in a pub making a "Sundowner" drink that he dubbed the "Kaylar Kalamity". Not long after, the crew in the pub were attacked during a Kaylar coup. Most survived, but several were injured. Their communications signals were being jammed, so with the help of Nano, he discovered the location of the jammer in the halls of the Parliamentary Assembly. He deactivated it, but was killed moments later when the Minister's aide, Talza, stabbed him in the back.
While on Talos IV, Pike experienced the killing again, this time done by Vina rather than Talza