Massa was a Duros male, identifiable by his green skin, who held the rank of admiral within the Alliance to Restore the Republic's fleet. He fought against the Galactic Empire during the Galactic Civil War. Shortly following the Battle of Hoth, Massa was present at a secret gathering of Alliance leaders in a Rebel safe house located on the planet Zastiga, where he discovered that the Empire was in the process of constructing a second Death Star orbiting the moon of Endor. When Admiral Gial Ackbar initially displayed a hologram of the new Death Star to those present, Princess Leia Organa initially confused the image with that of the original Death Star, but Massa corrected her, noting that it had been destroyed during the Battle of Yavin. After learning that it was an entirely new battle station, Massa expressed doubt about the fleet's ability to destroy the Death Star; however, the Battle of Endor later proved him wrong.
Jason Fry created Massa for The Essential Guide to Warfare, a Star Wars Legends sourcebook that came out in 2012 and identified Voon as the Duros's first name. The novel Moving Target: A Princess Leia Adventure, co-authored by Fry and Cecil Castellucci and published in 2015, confirmed the character's canon status. There is one instance where Massa's name is incorrectly spelled as Vassa.