Captain Shea Brennan was a main character in 1883: A Yellowstone Origin Story. A 75 year-old Union Civil War Veteran who lost his family to small pox. Shea was a no-nonsense, tough-as-nail's leader of the settler caravan travelling from Texas to Montana.
Shea was born in 1808 and grew up in the Northern United States, however little is known regarding his childhood. As a young man, he served in the Union Army where he rose to the rank of captain and fought in the American Civil War. He also met and befriended his fellow comrade Thomas, a sergeant of the Buffalo Regiment.
He met and fell in love with a woman named Helen where together they had a young daughter. Tragically they both contracted small pox and perished. Emotionally devastated, Shea set fire to their house and contemplated suicide.
Shea later became employed as a high-ranking member of the Pinkerton Detective Agency alongside Thomas who acted as his second-in-command. The pair were assigned the role of guiding a group of European immigrant settlers including the Dutton Family on an arduous journey from Texas to the untamed wilderness of Montana.