Casavir is the first mate of the Siren's Call, the pirate ship captained by Isabela.
Casavir is encountered along with Isabela at The Pearl in Denerim
. By initiating a conversation with him, the Warden can receive more information about Isabela.
During a conversation between Isabela and Fenris, it is revealed that Casavir was imprisoned in Val Chevin. She broke in to get him out, but left him on the docks afterwards.