She has dark red hair and eyes like Aviendha.  She looks a lot like Aviendha, only a little older and plumper.
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. She had tried to talk Aviendha out of becoming a Maiden, as she wanted Aviendha to marry.
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, a threat that she followed up on.
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.
After her time as gai'shain, she put off the white and returned to her husband.