"I'M MUCH MORE DERE THAN MIKU! AND AREN'T I THE CUTER ONE?"
Kagamine Rin is one half of the second installment of Vocaloid2, also known as Kagamine Rin/Len. She, as well as Len, were created by Crypton Future Media as singing synthesizer application software that could be used to cover music as well as create original songs. Their shared software included two separate voice banks, using the same singer's voice samples, because there was demand for another male Vocaloid while they were still creating Rin. Because of this, Rin and Len are considered both to be mirror images and twins, but, due to the user-created nature of the characters in which the fans decide on their personalities and other information, Crypton backpedaled on marketing them explicitly as twins as was originally intended. It is, however, official that she and Len are 14 years old in regards to human age.
Several months after release, an update was created for the Kagamine software, improving the vocal quality of the two voice banks. However, their voice banks remain only Japanese, unlike some later releases.
Before the Kagamine product was released, however, there was a debate between the fan community on what her signature item would be (such as Hatsune Miku's signature leek item; Kagamine Len wasn't yet announced at this time). All of the suggestions didn't have a very strong impact, including an orange, a banana, or an onion—although, later, Rin would be given an orange and a yellow onion, while Len would get the banana.
However, a video emerged on Nico Nico Douga titled "Excuse me… I want to make a reservation of Kagamine Rin…" In the video, the Hatsune Miku software was used to have the character Hatsune Miku give a small speech, introducing herself, and then bringing up the topic of Rin.
A loose translation of what she said would be, "Miku has the leek, doesn't she? In short, a vegetable! Therefore, I think that Rin… should have a road roller! How's that?" Because of the unexpected proposal of the video, the idea soon caught on, and the Road Roller became a character item for both Kagamine twins. This video was also the history of the Road Roller's signature sound effect, "WRYYYYYYN", as Miku also suggested, as it sounds like Rin's name. The entire idea was based on the character Dio's (of Jojo's Bizarre Adventure) infamous time-stopping attack.
As Rin's personality is decided by the fans, there have been many different interpretations of her. On top of that, many music videos place the characters in different situations and roles, but, as music videos are almost always fantasy even in the real world, these can be disregarded. However, there are recurring aspects of Rin's personality that can be pieced together with her most important songs and videos.
Rin is usually regarded as having something of a grudge against Miku, the more popular idol. She has been given the motto "OK, green is the enemy!" in numerous songs, "green" obviously referring to Miku. Her jealous side leads to her proclaiming things like how much cuter and nicer she is than Miku, as well as, "being cute doesn't get you anywhere, so I'm going to be tsun all the way!" Tsun is referring to the anime archetype tsundere, with tsun being the mean or cold exterior and dere being the nice interior of the character's nature.
Tsundere, despite Rin's use of it to make herself seem better than Miku, is actually a good term to describe her most of the time. She tends to treat the people she appreciates the most as if she doesn't like them at all, a very good example being her twin brother. This is probably due to the fact that, when it comes to showing appreciation, she gets very, very shy, as in the song "I Like You, I Love You" in which half of the lyrics are just her struggling to say the title, never quite getting all the way through until the very end.
However, rather than being a typical mean tsundere, she's actually more of the selfish, demanding type, and a few songs have a running gag of her demanding of her Master (whoever is using her software at the time) to give her snacks as a reward for singing, including one instance of her running him over with the Road Roller for skimping. In fact, in some videos, she's very quick to resort to running people (including her twin brother) over with the Road Roller when she is slighted, an indication of a quick temper.
As a Vocaloid, Rin can sing songs and play music videos.
So far, she has successfully been able to Bend maps of Nautilus and Bend open and close the "program" of the mini Road Roller that the city gave her after her first week there.
Hatsune Miku - Rin is jealous jealous jealous jealous that Miku is much more popular than her internationally. In Nautilus, Rin tends to lie and say that she is actually more popular than Miku. However, the two do still share a bond as fellow Vocaloids, and Rin tries to support Miku in whatever she does, even if Rin herself doesn't understand it.
Rion Steiner - This kid glitches a lot. Someone forgot to render one of his arms. Rin was the one that approached him while he was stuck in the network, and was mildly virus'd as a result. She couldn't bring herself to be upset with him, though, as he was, in her words, "a sick puppy."
During the High Event, Rin and Rion were… …somehow… they, um… well, anyway, afterwards, Rin freaked out and said that they should break up, and Rion, who had some extreme hangover going on, quickly agreed. And that offended her. Ever since, Rin has gone tsun full-throttle on him, including even leaving a cupcake for him that was filled with some unnamed gross thing. Despite the fact that Rin has never tasted anything, and, therefore, the cupcake she Bended was completely tasteless, neither of the two ever figured this out because Rion tossed it immediately because he doesn't like sweets. This offended Rin even more, and, when she let slip that there was something inside the cupcake, Rion was offended as well. Their relationship hasn't quite recovered since.
GlaDOS - Another program, but a very sarcastic one. She makes Rin a little uncomfortable.
Zack Fair - They are friends! Rin wants to help Zack as much as she can, and cheer him on.
Sephiroth - He's Zack's friend, right? Right?
Roxas - Rin and Roxas both manage to be naive about different things, but Rin tries to help explain what she knows about that Roxas does not. Rin believes Roxas is a "shota" and a "cute male", the error being that "males are not supposed to be cute," as he mentioned to her.
Ameras - He encouraged her idea to "boost morale" since she couldn't actually join the fight against Isis. The phrase has since become something of a catchphrase for Rin. He also gave her advice on how to help Miku get the human body she wants.
Sherlock Holmes - He was there for her first successful Bending: the map, which she used to direct him to the bar.
The Major - His intentions are questionable, but he seems to be on the "good side" for now…
Caren Velazquez - Caren drew fanart of her! Miku, too, but Rin was very flattered.
Xemnas - The first to unleash the tsun gates. In terms of the tsun:dere ratio it's like TSUN TSUN DERE TSUN DERE TSUN TSUN TSUN TSUN TSUN TSUN until you die from it.
Kanji Tatsumi - SEWING MUST BE POPULAR WITH BOYS, HUH, KANJI
Lelouch vi Britannia - Rin feels very appreciative towards Lelouch, as he encourages her and tells her that, despite her circumstances as being a program only inside the network, she can actually be helpful, albeit in different ways. He has also promised to gather a group to Bend her a body, although circumstances as of late have delayed this (lol i'm on hiatus)
Tony Stark - He removed the virus she caught from Rion.
Peter Jacob - THIS HORRIBLE MAN KILLS PUPPIES