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Fírnen's scales are described as the colors of forest leaves with the sun shining through them and the finest emeralds, becoming near white at the base of his paws. Unlike the other dragons, his eyes are said to not be the same color as his scales, but in fact amber. It is unknown how he perceives the colors in his vision, although greens are likely more vibrant than other colors. His spikes are depicted as being white although a darker shade than Saphira's.
At the end of Inheritance (book), Fírnen was still very young - only a few months old. His age is measured by his ability to breathe fire. When they finally met face to face, Eragon described Fírnen's mental voice as being very deep; even deeper than Glaedr's, Vrael's dragon Umaroth, or any of the Eldunarí.
Due to being raised away from everyone except Arya and animals for a longer time than Saphira, he is very unsure of what he can do until he meets Saphira, when she tested him for mating by fighting with him. He is described as having a sunlit, happy mind.
He hatched from the green Dragon egg, which was seized from Galbatorix's chamber at the end of Inheritance. By listening to the thoughts of the dragon inside the egg, Galbatorix determined that this dragon was male. This information was first revealed to Eragon by Murtagh at the Battle of the Burning Plains.
Fírnen hatched for the elf Arya when she had arrived at Tialdarí Hall and had been made queen. She raised him on the Crags of Tel'naeír (Oromis and Glaedr's old home) until he was able to defend himself. He and Arya visited Eragon before her coronation announcement and also stayed with him until his departure to find a new land in which to raise the dragons and train the Dragon Riders.
After meeting Saphira, the two fell in love and mated. They remained mates for a short time before Eragon left to find a new place for the dragons to be raised. Both Saphira and Fírnen were unhappy at their separation.
None of his lineage known as of now.
Saphira was his mate for a short time. This may be because Arya and Eragon had feelings for each other and these feelings were given to the dragons. It is unknown if Saphira will return to Fírnen or if they will lay eggs.
- Fírnen is not the only known dragon to have a different eye color than his scales. Shruikan and Vêrmund the Grim do as well, having pale blue eyes and red eyes respectively.
- He is the only dragon left in civilized Alagaësia as Thorn has left, wandering with Murtagh. Glaedr (at least in body) and Shruikan are dead and Saphira has left to raise the dragon eggs. It is unknown whether Saphira will lay eggs sired by Firnen.