Sora's Heartless

Sora's Heartless is the Heartless of Sora, and serves as a minor appearance of Kingdom Hearts and major appearance in Kingdom Hearts coded.

Kingdom Hearts
Kairi was revealed to be one of the Seven Princesses of Heart, but the door to Kingdom Hearts could not open because her heart lay within Sora. In order to return her heart and wake her up, Sora used the Keyblade of People's Hearts that Riku had to remove both their hearts, sacrificing his humanity and becoming a Heartless himself. This in turn created Roxas and Naminé, the Nobodies of Sora and Kairi respectively. When he managed to reunite with Kairi after a brief stint as a Shadow, Kairi used her light to return him to his human form. Since the Heartless he turned into was a Shadow, it would seem that Sora has little darkness in his heart, as the strength of a person's Heartless is measured by the darkness in their heart.

Kingdom Hearts coded
In Episode 7 Data Riku revealed to Data Sora that Sora's Heartless is the source of all Bug Block courrption in the Data World. It turns out that in Kingdom Hearts: Chain of Memories the entire record of Jiminy's Journal, along with Sora and his friend's memories, were all erased and then restored by Namine. The Chain of Memories was supposed to have been reconnected, but the notebook... The record was still erased. While Data Sora gained his powers in the data world, exploring other Data worlds and gaining new power, the data version of his heartless was corrupting the other data, changing it to make itself stronger. When Data Sora fought the Data Heartless in the Data world, he didn't release any Hearts, but the minds of the Heartless and they were absorbed into Sora's Heartless. While he absorbed the Heartless, his power span out of control, and it was about to enter the real world. When Data Sora fights Sora's Heartless, it changes his shape and becomes a dark shadow shaped Sora. When Sora's Heartless was about to win the fight, King Mickey appears and helps Data Sora defeat it.