Sasuke Uchiha (KH:RTD)

Sasuke is a former member of Akatsuki and currently rank XXIII in Organization XXI.

Akatsuki
Sasuke and his older brother Itachi were found by Madara Uchiha living on the streets of Traverse Town and Madara saw potential in both of them and decided to recruit them to his organization of mercenaries called Akatsuki.

As he grew older he developed a Sharingan eye like that of Madara and Itachi. He trained with his brother and learned to manipulate and create lightning and flames. When Madara felt he was ready he sent him to Agrabah on a mission to destroy Xirbealg the leader of Organization XXI. When he arrived at Agrabah he saw 2 men in black cloaks walking in the distance. He approached them and Nathan and Xirbealg asked who he was. Sasuke didn't respond but instead charged full speed at Xirbealg with a lightning element attack that was easily blocked. Nathan decided to test out his dark abilities and used a small portion of his power to send them to the dark realm where he asked Xirbealg to stay out of the fight. Sasuke and Nathan both fought evenly but when Sasuke summoned a dark keyblade Xirbealg got in his way instantly and told Nathan to go back to Twilight Town as he gave him a potion. Xirbealg and Sasuke both fought at full power and used lightning element techniques. Xirbealg absorbs one of Sasuke's sharingan and defeats him, convincing him to join the Organizatiion.

Organization XXI
Sasuke trained for hours with Xirbealg in the Twilight Town mansion's computer room until Xirbealg developed a Mangekyo Sharingan and Sasuke developed Twilight energy.