In Star Wars: The Empire Strikes Back, the astromech droid that travels with Luke Skywalker is R2-D2.
R2-D2 is a small R2-series astromech droid introduced in Star Wars in 1977. In The Empire Strikes Back, he accompanies Luke during his journey from Hoth to Dagobah and later Bespin. His repair skills and electronic tools are essential to several missions.
The droid communicates mainly through electronic whistles and beeps, while C-3PO often interprets his meaning. A common mix-up is to identify the gold protocol droid as Luke's principal droid companion; C-3PO travels with the group too, but R2-D2 is the astromech assigned to Luke's X-wing.
R2-D2 also appears throughout the original, prequel, and sequel trilogies, making him one of the franchise's most consistently recurring characters.