Sora's Heartless

Sora's Heartless is the Heartless of Sora, and serves as a minor character of Kingdom Hearts and major character 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 free Kairi's heart, sacrificing his humanity, heart and becoming a Heartless himself. This 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. Sora stated to Kairi that while a Heartless, he nearly forgot everything.

This event triggered a side effect, and created the Anti Form when Sora was given his new clothes and Drive Form abilities a year later in Kingdom Hearts II.

Kingdom Hearts manga
In the Kingdom Hearts manga, Kairi says she is able to tell him apart from the other Shadow Heartless, because she can feel that it is Sora him, and the way that Sora's calves are shaped as a Shadow.

Kingdom Hearts coded
In Episode 7, the journal revealed to Data Sora that Sora's Heartless is the source of all of the Bug Block corruption in the Data Worlds. 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 Naminé. The memories were supposed to have been reconnected, but the notebook's records were still erased. While Data Sora gained his powers in the Data Worlds, exploring other Data Worlds and gaining new power, the data version of Sora's Heartless was corrupting the other data, changing it to make itself stronger. When Data Sora fought the Data Sora's Heartless in the Data World, he didn't release any Hearts, but the minds of the Heartless, which 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 fought Sora's Heartless, it changed its shape and became a dark shadow shaped like Sora. When Sora's Heartless was about to win the fight, King Mickey appeared and helped the Data Sora defeat it.

Playing As Sora's Heartless in Kingdom Hearts
Sora is turned into a Shadow Heartless after his second fight with Riku. He is only in this form briefly, and can only jump and run around. However, due to Kairi's heart, he is able to recover his human form again. The time the player spends in this form is relatively short.

As a Shadow Heartless, Sora is unable to activate any of the lift nodes to bring him back to the ground floor in Hollow Bastion, and has to rely on jumping off the sides of the building. However, if he jumps too far and lands in the ravine below, the lift node activates to bring him back to ground level. If is pressed when Sora is in this form, Sora's menu cannot be accessed. This is similar to how you cannot access the start menu if you are in Shadow Form in Kingdom Hearts II.