Tyra wishes to have a legacy after her plans come to fruition and the divines are destroyed. If Firn survives she is worthy of carrying Tyra's legacy.
Firn appears as a slightly corporeal female Guardian. The divine energy she is formed of is slowly being drained into Morthal, causing her to became weaker and increasingly transparent.
She was placed amongst the 13 Reapers, an elite group Guardians chosen to carry mortalkin into a new age free of divine ones. Tyra sets them up to be killed, and those who live have the ability to overthrow their divine masters.