Marris Thornhill is an Aes Sedai of the Brown Ajah. She is also Black Ajah and is part of a Heart along with Atuan Larisett and Karale Sanghir. Her "one other" was said by Yukiri Haruna to be among the rebels.
Marris is described as being plump.
Marris is one of the weakest Aes Sedai, just a level above Daigian Moseneillin at 44(32).
She has no Warder.
Faolain Orande is sent to Marris to be punished.
Her fate is undisclosed. There exists the possibility that she was one of the sisters that managed to escape the Tower, though her Oaths to the Black Ajah hunters may have prevented her flight. If that is the case, she would have been executed with the rest of the Black sisters unable to escape.