An Imperial Shard of considerable experience, Ider Firenz was a veteran of Brankfall, Tochi, and Ottomas. This so-called "unholy trinity" of operations took place amidst events the Imperial Combine would later come to refer to as the Trinity Crisis.
Due to her extensive experience and broad skill set, Firenz was assigned to evaluate the newly graduated counterpart Mostrum Appatine as he transitioned from his MAGA duties on a probationary assessment period. The pair were sent to offer close protection services to Proconsul Chellis Bel-Osanda, and her guest Ambassador Ohl Ain of the Lem Bataan Confederation, during the ambassador's diplomatic visit to Fengrir (→SFTD).
During that mission Firenz and Appatine were attacked by renegade ex-counterpart Maber Castigon. It is evident from the encounter that Firenz was somehow linked to the events which resulted in Castigon being sent to prison, and it is more than likely that she contributed to that outcome. Believing that Castigon was making an attempt on Ambassador Ain's life, Firenz placed herself in harm's way. She was shot in the head by Castigon and died immediately (→SFTD).
It's hardly a spoiler to mention that Firenz was taken out of the running pretty early on in Steal from the Devil, which may have disappointed some readers. She was a character who demanded attention from the word go, and had a great deal of potential.
As can be seen clearly during her appearances in GWOTS, Firenz played a significant role not only in the Trinity Crisis, but also in the events which later brought Maber Castigon to justice. When the Trinity Crisis series begins this will of course be explored.
Naturally Firenz will not be paired with Appatine during those books since he would be about thirteen Solars old at the start of the Ottomas incident. She will instead be paired with a different counterpart. She will be a significant player, but since we know she can't die she will not be the protagonist.