C-3PO and R2-D2 are droid counterparts in the series and the only characters to be in all the films as well as several television shows.
C-3PO (or “3PO”) is a protocol droid built by Anakin Skywalker; he is programmed for over 30 secondary functions and is fluent in over seven million different forms of communication.
R2-D2 is an R2 series astromech droid owned by Anakin Skywalker. R2-D2 is a navigator for both Anakin Skywalker and Luke Skywalker.