Ivah was a rylleh before losing almost all it's Night and was sent to the surface to collect Night as a final humiliation disguised as an opportunity.
Cat recruited Ivah as her guide, and since that time Ivah has served as her right hand in most manners drow. It became Lord of the Silent Steps when fae granted it winter powers, which later mostly was lost when Sve Noc took over Night.
Ivah essentially leads Cat's Losara sigil as her second in command. Following Hainaut and the Kurosiv arc in book 7, it also stands to take over First Under Night after Cat.
Ivah is tall, blade-thin and has pale grey skin.
During Book 5, Ivah starts to wear the Losara sigil painted on it's face. Purple base, with a silver tree down the ridge of it's nose and encircling it's eyes to finish the pattern.
Ivah's eyes changes a lot during Book 4 and 5.
When first meeting Cat it's eyes are silvery, but in a tone that was less vivid than the other drow prisoner's Cat took. You could also see tiny hints of an original green eye colour.
As Lord of Silent Steps Ivah's eyes were deep blue.
Later Ivah's eyes become the mix of silver and pale blue that came with Winter-Night.