Queen Trisna is a vassal of Maccavia, and is the wife of Agathor Krex. She is very loyal to Agathor Krex, since she is his wife. You can get quest from her like any other vassal, and helping her in combat will improve her relation to you, and your relation to Maccavia.
At the start of the game Queen Trisna owns Forde.
Queen Trisna is a smart and well-educated person, a charismatic leader and a quite experienced swordswoman.