Late in Kingdom Hearts, Sora becomes a Heartless for a short time, before being restored to his human form. In Kingdom Hearts coded, Sora's Heartless becomes a separate character, a digital entity created from the journal record of Sora's time as a Heartless. This Heartless serves as the main antagonist of the game and the source of the Bug Blox. Unlike other Heartless, Sora's Heartless held onto its feelings, memories, and selfhood, and this unique trait is commented on by others.
Ansem the Wise noted that Sora was unique for being able to regain his human form without first having to destroy his Nobody counterpart Roxas; he theorized it was because of the immense love Sora had for others that allowed this normally impossible occurrence.
After Sora, Donald, and Goofy track Riku down to the Great Hall, they learn that he has used the incomplete Keyblade of heart to summon the Keyhole of Hollow Bastion. He reveals that Kairi's lost heart has been within Sora all along, and that it must be released in order to complete the Keyhole. He attempts to release Kairi's heart by using the Keyblade on Sora, but Sora parries at the last second, and he eventually defeats the possessed Riku.
Despite this, Kairi is still catatonic, and Sora is unable to seal the incomplete Keyhole. In order to save Kairi, Sora turns Riku's Keyblade on himself. This releases Kairi's heart from within Sora, reviving her, but this also completes the Keyhole and casts Sora into darkness, transforming him into a Heartless, and also producing his Nobody, Roxas, as well as Kairi's Nobody, Naminé. Sora transforms into a Shadow, as he does not have that much darkness in his heart, just as Kairi awakens, and the Heartless rapidly emerging from the Keyhole force Kairi, Donald, and Goofy to flee.
As a Shadow, Sora follows his friends through the Lift Stop to the Entrance Hall, where he sees them being overrun by other Shadow Heartless. He rushes to help his friends, and though Donald tries to attack him, Kairi recognizes him and embraces him. Her light and heart return Sora to his human form, and Sora later states that while he was a Shadow, he nearly forgot everything.
Kingdom Hearts Chain of Memories
When they first enter Castle Oblivion and learn of its alleged power over memories, Goofy reassures Sora by reminding him that he did not lose his memories when he became a Heartless. Axel later explains to Larxene that this strange trait is what interests him about Sora.
At the end of Sora's journey through Castle Oblivion, the witch Naminé puts him to sleep so that she can reconstruct his memory, which she had tampered with under coercion from Organization XIII. All of the contents of Jiminy's Journal are removed, a side-effect of the record of Sora's Heartless beginning to assert itself.
Kingdom Hearts II
One year later, Roxas dreams of when Sora was turned into a Heartless, as well as when Kairi restored his body. Not long afterward, Roxas reunites with Sora, and Sora reawakens as a complete being. Although he is whole, his time as a Heartless and the magic of his new clothes allow him to periodically assume the Heartless-based AntiForm.
Kingdom Hearts coded
As a symptom of the secrets that were hidden within Jiminy's Journal, a digital version of Sora's Heartless comes to be and, like Data-Sora, it evolves into something more than a simple facsimile. To that end, the Heartless absorbs both the Bug Blox, which spread throughout the Data Worlds, and the minds of the slain digital Heartless, increasing its power exponentially.
After clearing the Bug Blox from the data Hollow Bastion, the Journal alerts Data-Sora to the source of the Bugs: Sora's Heartless. Upon defeating Maleficent and Pete, the origins of Sora's Heartless are revealed, along with its intent to enter the real world to feed on hearts. Data-Sora attacks the real Sora's digitized Heartless, causing it to evolve into AntiSora before resuming its Darkside-like form. Data-Sora grows weary from this battle; just as Sora's Heartless is about to claim victory, King Mickey appears. Mickey's aid gives Data-Sora the means of destroying the Shadow that serves as his opponent's true form.
Sora's Heartless is physically identical to the common Shadow; its digital incarnation within Jiminy's Journal first appears in the form of a Darkside with multicolored circuitry[?] on its shoulders and chest, then as an AntiSora, then finally as the Shadow that serves as its true form.
In his Shadow form, Sora can only jump and run. He can no longer activate lift nodes, and unlike other Shadows, he cannot perform lunge attacks. However, if Sora falls too far in the Lift Stop, a lift node will automatically activate to bring him back up to the correct floor.
Notes and references