Tantive II
Tantive II was a CR90 corvette used by the Rebel Alliance during the Galactic Civil War. When Biggs Darklighter's X-wing broke down while carrying the recently intercepted schematics for the Death Star, the Tantive II, along with the Tantive I and the Tantive III, was sent to escort Lambda-class T-4a shuttles which would restore the X-wing's power generator.
It arrived from hyperspace to find Rebel R-22 Spearheads fighting Imperial Assault Gunboats which were attacking the X-wing. After helping finish off the Gunboats, the Tantive II waited until the X-wing was repaired and followed it into hyperspace.
Behind the scenes
The ship is, presumably, a sister ship to the corvette Tantive IV which is seen in Star Wars: Episode IV A New Hope, though it is not shown with the rest of the group in Star Wars: X-Wing and whether the Tantive IV was ever directly involved with the others is unclear.