Thelin th'Valrass was a half-Andorian/half-Aenar thaan who served in Starfleet in the 23rd century. Commander Thelin served as second-in-command of the USS Ticonderoga. One year after his posting, he was killed in an avalanche during a survey of an unexplored planet.
In an alternate timeline created by the death of Spock in 2237, Thelin became first officer aboard the USS Enterprise in 2264, and continued to serve in that capacity until 2269, when he resigned from Starfleet and join the Andorian Science Institute. In 2274, Thelin was engaged to Thali sh'Dani but the Eveste Elders would not allow them to mate so they broke up. Due to the trauma of this event, Thelin decided to rejoin Starfleet. He was subsequently promoted to captain and given command of the Enterprise at some point before engaging Khan Noonien Singh.
In the 2280s, Thelin was present at the destruction of the Enterprise by Commander Kruge and the ravaging of Earth by the Cetacean Probe. He also led the attempt to reclaim Earth's atmosphere aboard the USS Copernicus, but was stopped when the Klingons invaded Earth.
In the ensuing war, Thelin was promoted to admiral and led forces in combat, including a succesful campaign in the Archanis sector. He was then chosen to take the IKS Katai, a Klingon bird-of-prey which Thelin had captured, and deploy the Genesis Device on Praxis ending the war. After this action Thelin was considered a war criminal by the Klingons and was voluntarily extradited to the Klingon Empire for trial to maintain the new, fragile peace.