Gideon Hask is a human Imperial special-forces officer, introduced in Christie Golden's novel Battlefront II: Inferno Squad (Del Rey, 2017) as Inferno Squad's second-in-command under Iden Versio.
Inside the Dreamers
During the squad's infiltration of the partisan cell known as the Dreamers, Hask and Del Meeko posed as brothers, running supplies and repairing TIE fighters while gathering the intelligence that destroyed the group. He fought at Endor and carried out Operation Cinder missions afterwards without visible difficulty.
The one who stayed
When Versio and Meeko defected over the burning of Vardos, Hask did not. He was promoted into Versio's old rank, followed the surviving Empire to Jakku, and went into the Unknown Regions with the remnant that became the First Order. He is the control group in the game's central experiment: the same training, the same orders, the same atrocity, and a different answer.
The reckoning
In 34 ABY, Kylo Ren captured Del Meeko on Pillio, tortured Luke Skywalker's location out of him, and handed him to Hask, who executed him. Hask killed Iden Versio in the same campaign, and was killed in turn by his former teammates. Three people who trained together in one squad finished each other off across thirty years, which is a tidier piece of tragedy than a tie-in shooter had any obligation to attempt.