Costeau used to be a Chantry sister, but left the order following the death of the Divine at the Conclave. She joined the Freemen of the Dales, but she does not share the ideology of some of the other Freemen.
Costeau is leading a group of Freemen from the Veridium Mine. When some of Fairbanks' refugees, including Gertrude, broke into the mine to steal supplies, she imprisoned them. She ordered her men not to attack unless provoked. Due to a misunderstanding, when the Inquisitor enters the mine, she assumes Fairbanks sent them, and attacks the party; she is killed in the process. The Inquisitor then releases the prisoners.