An Andorian officer who served as first officer of the Enterprise in a timeline in which Spock had died as a child.
Thelin is an Andorian who holds the post of first officer aboard the Enterprise in an alternate history created after a journey through the Guardian of Forever. In that history Spock died as a boy on Vulcan, and Thelin has replaced him.
When Spock returns from the Guardian to find himself unknown, Thelin acknowledges that the original timeline must be restored, even though doing so means that he will cease to hold his post. He wishes Spock success as Spock travels back in time to correct history.