Tsia used to work for the French federal agency DCRI and was later assigned to Interpol. She became part of JTF-12, a team formed after 9/11 by Interpol that specialized in profiling terrorists, utilizing infiltration. The team consisted of Emily Prentiss, Sean McCallister, Clyde Easter, Jeremy Wolff, and herself. At some point after the team disbanded (for unspecified reasons), she and Jeremy went into a relationship and eventually got engaged.
When Ian Doyle, a terrorist she and the rest of the team captured, escaped from prison, killing a motorist and stealing his car in the process, Prentiss becomes concerned for her former teammates and contacts Tsia, who tells her that Doyle thinks that Lauren Reynolds, the alias Prentiss had to go by when she infiltrated Doyle's organization, is dead from a car accident. Later, Tsia realizes that Jeremy perished from an unspecified natural cause and calls Prentiss, who is confident that Doyle ordered Jeremy's hit and tells Tsia to go into hiding. Having no other options, Tsia does so.
After Sean McCallister and his family are killed by Doyle, Prentiss calls Tsia and Clyde to help her track down Doyle and kill him. The three of them communicate by disposable cell phones at a plaza; Tsia tells them she was devastated that she couldn't attend Jeremy's funeral. They tell Prentiss that that they started their quest of Doyle, having Tsia used her contact in Washington D.C. to track him through one of his alias and found out about his travel schedule. Using one of his aliases, Tsia and Clyde manage to find Doyle and tells Prentiss about this.
Prentiss starts to distrust Clyde because he didn't tell her that Doyle was imprisoned in North Korea and not in Russia. Finally believing that Clyde can no longer be trusted and that he is hiding information to them, Prentiss instructs Tsia to meet her at an apartment room belonging to a forger who created fake IDs for Doyle's son Declan and Declan's nanny. However, when Tsia arrives, she is shot in the forehead through the door and killed by Doyle, thus leading to the belief that Clyde is the mole. When the BAU discover Tsia's body, Prentiss is devastated that she led her dear friend to her murder and even threw up outside.
It was revealed in the episode that it was in fact Jeremy who was Doyle's informant. Knowing this, Clyde withheld information from Tsia, who was Jeremy's fiancée and therefore could have been involved in the matter.