Note: Sergeant Joanna will only be encountered if you imported an Awakening save where the Warden defended Vigil's Keep and allowed Amaranthine to be destroyed, or if you used the "No Compromises" pre-built history.
Sergeant Joanna is involved in the quest The Conspirators. She offers the job to Hawke after trying to get permission from Marlowe Dumar with no success. She then gives them a list of names and descriptions and states that she'll reward Hawke if any of the listed persons are killed. She later reminds them that despite being given the job, Sergeant Joanna might beat them at killing the conspirators. Once they are killed, she returns home.