She has dark red hair.
Niella was not a Far Dareis Mai, like her sister, but a weaver, taken as gai'shain by Chareen Maidens during a raid on Sulara Hold. She avoids Aviendha when she can, as her sister seems to be very strict with her.
After Rand and Aviendha returned to Rand's bedchamber after sleeping together in far off snows, Asmodean mentions that a gai'shain looking very much like Aviendha had come requesting that Rand eat. Asmodean had turned her away, saying that Rand and Aviendha would like time alone. Niella had whooped and laughed, and ran off to tell the Maidens. Aviendha vowed that she would leave a welt on her sister's bottom for every Maiden she told.
In Cairhien, Egwene ran into Niella exiting Rand al'Thor's bedchamber with an armful of dirty bedding. Egwene's attempt to wring information about Rand's return fell on deaf ears - Niella insisted on leaving to avoid being switched by Aviendha if she found her standing about with Rand's dirty linen.