While the Star Wars franchise, to this point - when looking at the movies at least - the fate of the Galaxy has been presented in a way that shows it relies on the big heroes, on the Jedi, on Luke, on Rey, and a small core group of three or four people. In reality, there are a variety of unsung heroes, not least of all R2-D2, and even his protocol counterpart C-3PO.
The two are a staple in the franchise, appearing in every movie bar one, in both animated shows - admittedly in some of their worst episodes - and in a variety of comics, and throughout this, both in-universe and out, the pair continue to get better.
Meeting Each Other
After somewhat surprisingly getting revealed to be the creation of Anakin Skywalker, C-3PO did not meet R2-D2 for a good while and was not entirely finished when they did meet.
But when they met and instantly hit it off with a classic dynamic in The Phantom Menace via working on Anakin's racer, a beautiful friendship blossomed, and a partnership that would last for decades began.
Threepio's Upgrades
As mentioned, Threepio was not fully completed when we first chronologically meet him in the films, that did quickly change, however, and Threepio underwent many upgrades over the years.
He got his gold plating, and eventually got it looking much cleaner and sleeker with new limbs and all. On top of that, he had various pieces of internal equipment upgraded, including his TrangLang III communication module, by the time of the sequels, allowing him to speak over seven million languages.
Part Of The Rebellion Under Ownership Of Bail Organa
Following the death of Padmé Amidala, and the fall of Anakin Skywalker, Threepio and Artoo fell into the hands of Bail Organa, and as such a part of the rebellion.
As seen in Rebels, Artoo and Threepio went on many missions for Bail, and by coming under his ownership, they got passed on to Leia, allowing them to form many a friendship, including one with Leia.
Artoo Coming Into The Service Of Padmé
Long before they belonged to Leia, though, Artoo and Threepio belonged to her mother, Padmé, with a combination of luck and skill, ensuring a place by her side for Artoo.
As shown in The Phantom Menace, R2-D2 was the only surviving droid of a group who was attempting to fix the shields of Padmé's ship. Threepio did so, allowing the ship to escape the blockade into hyperspace, kicking off his entire Star Wars journey.
Threepio Came Back Into Possession Of Anakin & Then Padmé
As mentioned, though, it was not only Artoo who belonged to Padmé but Threepio too, who had a far less adventurous route to becoming so.
He was left with Shmi on Tatooine when Anakin left, and therefore owned by Cliegg and the Lars' before Anakin came back for his mother. Threepio left the planet with Anakin, Padmé, and Threepio and became a good friend of Padmé under her ownership.
Their Ever-Growing Friendship
One of the highlights of The Rise Of Skywalker was C-3PO, and perhaps his best line in the movie is when he tells Artoo that he is his best friend.
It is a genuinely emotional moment; these two characters had been together for decades, and grew together, bickered with each other, been heroes together. This was the first time we saw them admit their true love for each other; love fans can see in just about every appearance from them.
Going After Obi-Wan Kenobi (& Luke Skywalker)
The actual introduction to the characters came in A New Hope as they are sent on a mission to find Obi-Wan Kenobi on Tatooine for Leia.
Things do not go as smoothly as simply finding Obi-Wan for the droids, which was for the best as they ended up in possession of Luke Skywalker, eventually getting Obi-Wan, and headed off for another grand adventure, this one resulting in the rescue of Leia, the destruction of the Death Star, and the eventual saving of the Galaxy.
Artoo's Relationships With Anakin & Luke
One of the best friendships many fans point to from the films is that of Luke and Artoo. They are an excellent pair, and there is a genuine emotional connection there, with both seemingly improving the life of the other.
Long before Artoo met Luke, he had another non-droid best friend, that being Anakin. Anakin's attachment to Artoo was incredibly apparent throughout The Clone Wars, and it was a touching friendship that also gave way for the friendship of Artoo and Ahsoka. He is a popular little astromech.
Constant Ability To Survive
Artoo and Threepio go on countless adventures across their time in the Skywalker saga, from the Battle of Geonosis to the Battle of Endor, from trips to Dagobah to escaping from Hoth, the pair are always in and around danger.
Yet, despite 'only' being droids, the two are constantly able to defy odds and survive these life-threatening situations together, and time and time again live on to entertain fans some more.
Heroes
As mentioned earlier, the two are heroes and unsung ones at that. Like Chewbacca, Artoo and Threepio get nowhere near the amount of credit they deserve, especially Artoo.
Always helping out in crucial situations, holding his own against fellow droids or enemies, and performing rescue missions when called upon, Artoo is as much of a hero as anybody else, and a lot of the time, Threepio is right there by his side.