Tessa's physical appearance was never described in detail. She likely has the same bright red hair as several other members of her sect are described as having.
Little is known about Tessa, although she seemed to fit in with other members of her faction well enough. She generally pairs up with Conall, although the exact nature of their relationship was never described.
Conall and Tessa were put in charge of keeping control of the media in Atreya, which they managed to do for the most part. Their only major lapses were when they were called away to take part in operation that became the Battle of Rathmore's Gate, and the attempted Destruction of Harold. Her current fate is unknown as her reaper was not mentioned as being killed.
Tessa had the power of destruction, which made her difficult to deal with in a large group since she could provide hard-hitting covering fire for her allies. This was ultimately not enough to save her from Harper, however.