Finn is the brother of Aspen Matthews. When they were children they left their home beneath the sea after witnessing the murder of their parents. The ship they left in was destroyed but Aspen made it to safety. What happened to Finn is unsure, but he ended up unable to revert to his flesh form. The Blue are able to turn from flesh and bone to water and back again, often using this to escape danger, but Finn was trapped in his water body and can't make himself whole again.
After the trickery of Killian disguised as Finn, Aspen and Chance Calloway rescued her real brother in a facility being prisoned by Chance's brother, Chris Calloway.