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R2-S4 was an R2-series astromech droid given to Pike Angeles to help him conduct tests on the modifications he was applying to the Rebel Alliance's first fleet of Y-wings on the planet Dantooine. Pike kept R2-S4 even after these tests were complete, however, bringing the droid into the field with him. Because R2-S4 was adept at breaking into then controlling spaceships onboard computers, particularly those of Imperial-model small or medium starships and transports, Pike would often bring him along in his attempts to steal starships. R2-S4, like most R2 units, was very independent and would tend to wander off whenever he was bored.