C-3PO is a fictional male Robot translator in the Star Wars series. The character was portrayed on screen by Anthony Daniels. His allegiance / loyalty is to The Old Republic. Lets first hear C-3PO describe himself 'I am C-3PO, human cyborg relation'. C-3PO is commonly referred to as just Threepio for short.
Ok, that's enough. C-3PO is a humanoid robot that was built by Anakin Skywalker when Anakin was a child slave on the planet of Tatooine. At the beginning, C-3PO had no outer skin, he was just wires and components but as the prequels progressed, the skin was added. In the prequels, C-3PO is a bit character unlike his near central role in the star Wars IV - A New Hope. C-3PO is capable of translating six million languages except the language used by the droid on Hoth. He is one of only two characters to have appeared in all prequel and originals if you exclude the ghost of Obi-Wan Kenobi and Anakin Skywalker / Darth Vader.
As part of the sell that George Lucas did to movie executive were drawings of how Star Wars might look. Some of the pictures featured the two robots.
C-3PO friendship with R2-D2 is on par with Han Solo and Chewbacca. Their friendship wasn't that strong in the prequels, they took a while for the friendship to take off. The two robots first meet in Phantom Menace but its only a brief encouner between the two.
The friendship is really strong between the two in the originals. The first film, New Hope was seen through their eyes as they escape from Tantive IV which had been invaded by Dath Vader. They act like squabbling friends at the start of the film, each wanting to go their way which they do but soon join back up with one another when they are caught by Jawa scavengers.
C-3PO has an emotional bond with his smaller friend, when R2-D2 is to be Luke Skywalkers co-pilot, C-3PO wishes him to come back safe. In episode V, they are separated when Han Solo takes C-3PO to Bespin via the asteroid belt and R2-D2 goes with Luke in the X-Wing.
Both robots are sent together to barter for Han Solo's life as he is frozen in carbonite by Jabba, the Hutt.
In the first film, C-3PO is just components and doesn't do much. C-3PO does meet R2-D2 but they don't interact much as Obi-Wan Kenobi, Qui-Gon Jinn and Padme Amidala leave C-3PO on the planet when they leave with their new Jedi apprentice, Anakin Skywalker.
C-3PO is still on Tatooine and when Anakin Skywalker and Padme Amidala come to the planet to look for Shmi Skywalker (Anakin's Mother), C-3PO manages to get off the planet with Anakin Skywalker. During the battle of Geonosis, C-3PO wanders round doing absolutely nothing of use. C-3POs head gets knocked off and ends up on a Trade Federations droid.
Still just a bit character, he is now the possession of Padme Amidala and is aware of her pregnancy. C-3PO travels with Padme Amida and Obi-Wan Kenobi to Mustafar to try to persuade Anakin Skywalker that what he is doing is wrong. Both of the droids stay out of the fight between Anakin and Obi-Wan Kenobi. The droids memory are wiped so they don't remember that they were once owned by Anakin Skywalker who has now become Darth Vader. C-3PO has the last words said in the prequels and the first words in the originals.
C-3PO and R2-D2 are onboard the Tantive IV space craft flying above Tatooine when an Imperial Star Destroyer captures the craft and Darth Vader boards with his Stormtroopers. C-3PO and R2-D2 escape in an escape pod to the surface below. Their mission is to get a secret message to Ben Kenobi, better known as Obi-Wan Kenobi. Having crashed landed on the planet below, they wander the desert until they are captured by Jawas.
The two droids are sold to Lars and Beru Owen whose adopted son Luke Skywalker wants to get away from the farm. When R2-D2 shows Luke the message, Luke Skywalkers adventures begin. With their new master, C-3PO and R2-D2 travel to Mos Eisley after having found Obi-Wan Kenobi.
In Mos Eisley, the travellers meet Han Solo and Chewbacca who agree to take the party to Alderaan. During the flight , the planet, Alderaan is blown up and the space ship they are in, the Millennium Falcon is dragged inside the Death Star. The two droids stay out the way whilst Han Solo, Luke Skywalker and Chewbacca rescue the Princess Leia. The droid leave the Death Star space station inside the Millennium Falcon and travel to Yavin IV where the Rebel Alliance are hiding from the Empire. Whilst R2-D2 takes part in the battle against the Death Star, C-3PO stays at the base.
C-3PO is with the Rebel Alliance on the planet of Hoth, hiding from the Empire. When the call comes to evacuate, C-3PO leaves the planet inside the Millennium Falcon. The Falcon travels to Bespin via hiding in the stomach of a giant worm in an asteroid field. Whilst on Bespin, C-3PO discovers Stormtroopers on the planet and before he can warn anyone, he is destroyed. C-3POs parts are recovered by Chewbacca who puts him together. Lando Calrissian has betrayed the Rebels and has handed Han Solo over to the bounty hunter Boba Fett for delivery to Jabba, the Hutt. Lando changes sides again and releases the Rebels and helps them to escape. C-3PO escapes with the Rebels onboard the Millennium Falcon.
C-3PO and R2-D2 visit Jabba's Palace to bargain for the release of Han Solo. Unbeknown to the two droids, Luke Skywalker's message offers the droids for the release of Han Solo. Jabba uses the robots as servants. C-3PO provides translation services for Jabba and R2-D2 is nothing more than a drinks servant. When Luke Skywalker causes mayhem and disorder on the hovership, the two robots make an exit but fall into the sand.
Han Solo, Princess Leia, Lando Calrissian, Chewbacca and C-3PO head to the Rebel fleet where C-3PO is to join the assault on the shield generator on the Moon of Endor led by Han Solo. During the time on the moon of Endor, C-3PO gets worshipped as a God by Ewoks because of his golden complexion. The Ewok release the Rebel assault team and make friends. For the battle on Endor, the two droids stay out of the fighting, they avoid the battle.
Along with R2-D2, C-3PO returns in Star Wars - The Force Awakens, this time he is sporting a red arm which he uses to comedic affect when Han Solo is reunited with Princess Leia, he introduces himself and says who he is just in case he wasn't recognised because of his red arm. It is not explained as to how he got his red arm, we'll probably find out more about how it came to be later in sequels.
His friend R2-D2 is in a sleep mode, waiting for the return of his master Luke Skywalker. C-3PO stays on D'Qar with the rest of the Rebel Alliance.