Senestra, also known as Senestra the Snake, is a Tevinter loan-shark operating in Kirkwall. She can be found on the west side of the Docks at night, surrounded by her henchmen.
Involvement
Senestra is involved in the quest A Debt in the Family. If Hawke had Keran be ejected from the Templar Order, Keran's family eventually falls into debt with Senestra.
Another templar recruit, Margitte requests that Hawke to negotiate with Senestra on Keran's behalf. When confronted however Senestra however decides that selling Hawke's head would be more profitable than any negotiated settlement and will attack Hawke, forcing Hawke to kill her.
Alternateively if Keran remained a Templar, Senestra can still be encountered at the Docks but will ignore Hawke entirely.