A PROXY droid, or a holodroid was a droid unit infused with sophisticated holographic technology. Holodroids could change their appearance and copy abilities at will. In this regard, they were similar to TC-SC infiltration droids and the Jedi Training Droids.
One such holodroid was PROXY, companion of Galen Marek around the time of the Galactic Civil War. Following his death, a clone of Galen Marek was created on Kamino with the purpose of eliminating the Rebel Alliance. As part of his training, Darth Vader ordered the clone to destroy a trio of Sith training holodroids imitating Juno Eclipse and a pair of Rebel troopers in order to test the clone's willingness to destroy his template's legacy. The clone easily eliminated the droids disguised as Alliance troops, but the imprinted memories and emotions of his progenitor prevented him from striking down the one disguised as Eclipse. Disappointed with the result of the test, Vader destroyed the droid and determined that the clone was another failure, no different than the other clones that couldn't free themselves from Marek's emotions.