Riku is one of several original characters appearing in Kingdom Hearts, Kingdom Hearts: Chain of Memories, Kingdom Hearts 358/2 Days, Kingdom Hearts II, and also one of the main characters in the series. Riku lives on Destiny Islands. He is the best friend of Sora and Kairi. He is 15 at the start of Kingdom Hearts and Kingdom Hearts: Chain of Memories, and 16 during Kingdom Hearts II. Since He would be reappearing in Kingdom Hearts Birth by Sleep, he would be 5 years old then since Kingdom Hearts Birth by Sleep is set 10 years before the start of Kingdom Hearts itself. Riku's name is derived from the Japanese word for "land."
In Kingdom Hearts, Riku is a calm, cool, collected teen who is not afraid to go far beyond his limits. This shows through when he shows no fear in stepping into the dark portal when Destiny Islands was being consumed by darkness, along with his statement "I'm not afraid of the darkness!" Also, he seems to be aware of Sora's romantic feelings toward Kairi and often teases him for liking their friend, such as when he challenges Sora to a race, Sora saying that if he wins, he gets to become captain of the raft they're constructing; Riku ups the ante by saying that if he wins, he gets to share a paopu fruit with Kairi.
He also showed a strong competitive streak, along with a bit of jealousy. He shows his competitive side when he challenges Sora, saying that his heart is stronger than Sora's heart. It is not actually said aloud, but thanks to Maleficent, he became increasingly jealous of Sora. It started with him finding out about Sora's new friends, and his thinking that he had been replaced. The jealousy grew when he learned about Sora possessing the Keyblade. The jealousy turned into rage, resulting in the battles against Sora throughout Kingdom Hearts, and his possession by Xehanort's Heartless.
Though his traits were not too favorable in Kingdom Hearts, he becomes more heroic in Chain of Memories. Riku has many realizations. One of these was him realizing that he let his heart become weak. From that moment on, he began to shy away from the darkness in his heart. Through the scores of fights and battles in Castle Oblivion, he makes another big realization: he can't get rid of the darkness in his heart, and decides to use it without turning from the light like before. He matured immensely, and along with leaving his past behind him, he also left a lot of his unfortunate traits.
In Kingdom Hearts II, he shows a great amount of loyalty and care for his friends. To save Sora, he did everything he could to bring Roxas to DiZ to make Sora whole again, going so far as to give up his physical form for that of Xehanort's Heartless. As Sora journeyed on between the worlds, he dropped him some valuable clues, such as the sea-salt ice cream and the picture.
When he finally is found in the World That Never Was, he repeatedly shows regret, telling Sora that "he didn't want to be found," and almost refusing to come back to Destiny Islands because of his wrongdoings.
Ever since he was young, he had been curious with everything that is outside of his world. Over time, his curiosity became obsession and he began to think of his world as a prison surrounded by water. When the Heartless came, he led them to the Keyhole of Destiny Islands in exchange for a trip to another world. This obsession greatly diminishes throughout the year of Sora's slumber, most likely because he had already seen many other worlds that the curiosity disappeared. When he returns to Destiny Islands in Kingdom Hearts II, he still comments it as a small world, but agrees with Sora's remark that it is only a part of one that's much bigger. This marks that Riku has a better view of his home world rather than imagine as a prison.
Another good personality trait he showed was a strong protectiveness over his friends. He never wanted Sora and Kairi to find out about the time he spent in the darkness. He nearly sacrificed himself to protect Sora from Xemnas' attacks, leaving him injured after the final battle.
Though Riku matures significantly over the course of the series, he still seems to retain some of his old Kingdom Hearts self, particularly his sense of humor; he refers to Sora, Donald, and Goofy as "wise guys" when talking to the Emperor, cracks a joke about Donald and Goofy's intelligence at their expense ("Sora never did pick the brightest friends."), and calls Sora a "total sap" when going to confront Xemnas.
Riku stands with features similar to other characters. His hair is silver mimicking characters such as Sephiroth. He has aqua eyes (which turn yellow when he becomes Xehanort's Heartless form.) and has a more pale complexion like Kairi compared to Sora. Riku also is more muscular than Sora showing how at first it seems that he is "better at stuff" than Sora. Riku's appearance changes more with his clothes than his actual physical appearance as he only holds two major changes. In Kingdom Hearts II he shows these changes as he becomes darkness itself in the form of Xehanort's Heartless. This results from him having to transform to defeat and capture Roxas. Near the final battle of Kingdom Hearts II against Xemnas, Riku returns to his normal form except with longer hair and surprisingly paler eyes. He also has more defined features and is much taller. He has four different outfits, his first being a yellow shirt mimicking a tank top with baggy blue jeans connected to suspenders with large shoes (in keeping up with Sora and Kairi's feet apperance). This outfit is changed when Riku turns to his normal form in Kingdom Hearts II; this time he has a black zip-up tank-top and a white and yellow vest over it, his pants are a pair of baggy blue denim jeans, and his feet down to a more normal size with a simple pair of sneakers. Also in his Kingdom Hearts II attire, there appears to be a white band on his left wrist, but it isn't clear if it is for injury or just part of his outfit. Riku's outfit in Dark Form seems to resemble that of Xehanort's Apprentice's outfit. It holds the Heartless emblem on the chest and is a mixture of blue and purple with a white grass-like skirt on the bottom. Riku's last clothing change is at the end of Chain of Memories and the majority of Kingdom Hearts II as he wears the Black Cloak worn often by other characters. When he is not in the guise of Xehanort's Heartless, he wears a blindfold to suppress his darkness.