Eyllwe is named for a location in the "Throne of Glass" series, in which the Thirteen and their wyverns battle. Notably, the character her mother is named after was included in this storyline [Narene].
Eyllwe and her littermate Morath were raised in seclusion in the depths of the packs territory. No wolf knew anything about the pups sire - but none would dare ask Narene who fathered her pups.
Much like their mother, these two remain strong but elusive presences in the pack. Very few wolves - other than their sisters - can approach them freely. This pair likely would've been driven out when they reached adulthood, if not for the former lead Cloudjumper taking a liking to the then newborn Cursebreaker; their younger sister from Narene's 3rd litter.
When Cursebreaker took over as lead, and their sister Illyria freshly born, Eyllwe & Morath's place in the pack became something solid and unmovable. Narene's daughters would no longer be living on the outskirts of the pack, but necessary to its function and survival. Eyllwe makes for a fierce and cunning hunter, and is said to possess Foresight abilities. Although now welcomed in the pack Eyllwe choses to spend most of her time in isolation.