Yes. Riku will probably always have darkness in his heart, but it won't always overwhelm him. Riku learns, from King Mickey, that light is always present, and that he can't be afraid of the darkness. Riku harnesses the darkness and uses the power to his advantage.
Ansem persuaded Riku to submit to darkness by convincing him that darkness is superior. Because of Ansem's dark influence, Riku is most likely stuck with a dark heart, although he learns to control and use it to his advantage.
Riku was supposed to be the Keyblade Weilder, but he had darkness in his heart.
No, Riku was never in Organization XIII. He wore the cloak because he wanted to hide the darkness in his heart.
SPOILERSRiku never became a Heartless, because he never vanquished his heart but simply submitted to the darkness. His heart was enveloped in darkness and hid Ansem's appearance deep within it. Riku hid the darkness with his black blindfold to prevent Ansem's power from fully overtaking Riku, but when Riku needed to defeat Roxas at Memory's Skyscraper, he removed the blindfold and revealed Riku-Ansem from within.
Unlike Riku, Sora and Kairi didn't have darkness laid dormant in their hearts. Riku needed the blindfold to retain the darkness in his heart that Ansem had created from Kingdom Hearts 1.
Since there is a Nobody when there's Heartless, Sora's Heartless "Anti-Sora" wields darkness, since Heartless are beings of darkness. Otherwise, Riku-Ansem is the one who wields darkness, because Riku had released the darkness in his heart to use it.
Ansem is controlling Riku's body while Riku still has very little control of his heart.
Yes and no. He isn't necessarily an enemy, but he is a protagonist throughout the majority of the game, due to Maleficent's dark influence and the darkness in his heart. He is still Sora's friend, just under control by Ansem. Riku always struggles with darkness, but is nothing close to an enemy after the events of the first Kingdom Hearts.
The Oblivion is made of darkness and light and colored black. Riku is consumed by the darkness, so the Oblivion resembles how Riku is.
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Ansem/Maleficent convinves Riku to give into darkness so that he may obtain dark powers which he thinks he can use to regain Kairi's heart. However, Ansem is really just using Riku as a host. So technically, he doesn't turn into Ansem; the two are still different people.
DiZ needed Roxas to awaken Sora, so Riku awaited Roxas in Memory's Skyscraper. When Roxas fought and defeated Riku, Riku had to release his heart's darkness and assume Xehanort's Heartless' appearance to finally restrain and capture Roxas.