The "Outlander" is a title used by an individual who was captured by the Eternal Empire after the fall of the Order of Revan. While investigating Wild Space with Darth Marr, the command ship they were on was boarded and destroyed by Skytroopers, trapping them both and preventing them from reaching escape pods before the ship's destruction.
After their capture, the individual was blamed for the murder of Valkorion and frozen in carbonite. The Outlander was awoken five years later by Lana Beniko and T7-O1, and escaped Zakuul with the help of Koth Vortena. This individual would later create a large Alliance as a means of destroying Arcann and the Eternal Empire.
The actual title "Outlander" was first used by Knights of Zakuul while the individual escaping with Lana Beniko fought through Zakuul in an escape attempt. The title stuck later, as almost nobody was able to confirm the gender or species of the individual.