In 2154, Thalen assisted Degra in testing the second prototype of the Xindi weapon in the Calindra system. Their work was interrupted when the AndorianwarshipKumari stole the prototype, which was later destroyed by CaptainArcher of the Enterprise. (ENT: "Proving Ground") Shortly thereafter, Thalen was captured along with Degra by Archer and the crew of Enterprise to gain information as to the whereabouts of the weapon. His voice was duplicated in an attempt to trick Degra into revealing the coordinates of the Azati Prime system. His memory was erased by Phlox so he would not remember the incident, or that Degra had unknowingly revealed the location of the weapon. (ENT: "Stratagem")
In the final draft script of "Proving Ground", Thalen wasn't named, only ever referred to as Degra's "assistant". Though an initial scene description in the final draft script of "Stratagem" indirectly referred to Thalen as one of the assistants who were to reappear from "Proving Ground", a bracketed note immediately thereafter stated about him specifically, "One of them will henceforth be known as Thalen."
It is possible that a Primate who appeared in "The Council", who was dismissed by Degra when a Sphere-Builder appeared on the bridge, was Thalen as well. However, there was no cast credit for Josh Drennen or Chris Goodman in the episode and there was no indication it was Thalen.