Nephelle, as Azriel explains to Feyre, was a seraphim (a genetic cousin of the Illyrians) with defectively small wings. In her early life she trained but was never allowed to fight because she was a weakness in the flying armies.
Nephelle is one of Miryam closest friends and saved Miryam's life when the sea that had parted was about to come down and drown Miryam who couldn’t move due to a spear in the chest. The daring rescue shocked everyone because she had been rejected from the Seraphim army because her wings were too small to hold the army lines without creating gaps and she was of a slight build not strong enough to even hold the Seraphim shield.
She scooped up Miryam and flew her though the rocks and ocean bottom at an unmatched speed. Her small wings made it easy for her to navigate through the sharp rocks as the waves fell down around her when another of the army would have been stuck.
Suffice to say her lover, who was one of Drakon's Generals, made Nephelle her wife that night.
Azriel tells Feyre that the story became known as the Nephelle philosophy, that the smallest wings might be the saving piece to a war. Since the day of the battle, the people of Cretea hold an annual flying race and the winner is crowned by Nephelle and her wife.
- A Court of Wings and Ruin (mentioned)