Rage... Hatred... Glitches to feed my Darkness... And soon, Hearts to sate my hunger!
Sora's Heartless is a minor character and a form that Sora takes during the final act of Kingdom Hearts. In Kingdom Hearts coded, the digital version of this Heartless serves as the main antagonist and the source of the Bug Blox corruption. Unlike other Heartless, Sora's Heartless held onto his feelings, memories, and selfhood as a Heartless, and this unique trait is commented on by others.
The physical Sora's Heartless which appeared in Kingdom Hearts originally took the form of a Shadow, but when Kairi recognized him and hugged him close to her heart, the pure light of her heart restored Sora to his human form, but without his Nobody, Roxas, Sora was still incomplete.
The digital Heartless within the Journal, which appeared in Kingdom Hearts coded, strongly resembles the Darkside Heartless, as it has short legs, and long, muscular arms. It has a large heart shaped hole in its abdomen, which penetrates straight through from the front to the back. Sora's Heartless's head is covered in dozens of twisted, black tentacles and only its small yellow eyes - a trademark feature of any Heartless - are visible. Two small vestigial wings protrude from the creature's back. Sora's Heartless has black skin, but, unlike Darkside, its shoulders are adorned with yellow and red symbols, which closely resemble the designs found on the Bug Blox. It later appeared as an AntiSora, and finally as a Shadow.
After Sora, Donald, and Goofy track Riku down to the Great Hall, they learn that he has been possessed by Ansem and used the incomplete Keyblade of People's Hearts 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 moves to release Kairi's heart by using the Keyblade on Sora, but Sora parries at the last second, and eventually drive off Ansem.
However, with Sora unable to seal the incomplete Keyhole, he resolves to use the Keyblade of People's Hearts on himself. This releases Kairi's heart from Sora, reviving her, but also completes the Keyhole and casts Sora into darkness, transforming him into a Heartless while creating Nobody Roxas. Sora fades into a Shadow, as he does not have that much darkness in his heart, just as Kairi awakens, and the Heartless rapidly emerge from the Keyhole, forcing 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 (in the manga, Kairi jokes that she recognized Sora's Heartless by the shape of its calves). Her light and heart return Sora to his human form, and Sora later states that while a Shadow, he nearly forgot everything.
When they first enter Castle Oblivion and learn of its alleged power over memories, Goofy reassures Sora by reminding him that he didn't lose his memories when he became a Heartless. Axel later explains to Larxene that this strange trait is what interests him about Sora. Sora and his friends eventually discover that the witch, Naminé, somehow has the ability to manipulate memories, allowing her to manipulate the memories of Sora and his friends.
Organization XIII are forcing her to do this in order to brainwash Sora at Castle Oblivion, but Sora defeats the Organization members within the castle and rescues Naminé. They agree that Sora will be put to sleep so that Naminé can restore his jumbled memories, and after Naminé puts Sora to sleep, she blanks the contents of Jiminy's Journal, as well as Sora and his friends' memories, then begins restoring them. However, while the memories were reconnected, the Journal itself remained blank. This is later revealed to be due to the record of Sora's Heartless beginning to assert itself.
My... Nobody?! But that's crazy! I never turned into a Heart--oh... Right."
"It was when you came to rescue me, remember?
Sora and Kairi
One year later, the Replica of Sora's Nobody, Xion, is reunited with his Nobody, Roxas, and the pivotal creation of Sora's Heartless later appears within Roxas's final dream-memories. Finally, 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 allows him to infrequently assume the Heartless-based Anti Form.
As a symptom of the secrets that were hidden within the journal, the digital recording of Sora's Heartless gained new life and, like Data-Sora, evolved into 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 a source of the Bugs: Sora's Heartless. Upon defeating Maleficent and Pete, Sora's Heartless's origins are revealed along with its intent to enter the real world to feed on hearts.
Data-Sora attacks the digitized Sora-Heartless, causing it to evolve into AntiSora before resuming its Darkside-like form as Data Sora is worn out from the fight. Just as the Heartless is about to win the fight, King Mickey appears and helps Data-Sora slay it by holding the vessel in place so the latter can destroy the Shadow that is his opponent's true form.
- Riku (Ansem): "It’s no use. That girl has lost her heart. She cannot wake up...The Keyhole cannot be completed so as long as the last princess of heart still sleeps...Yes, and without her power, the Keyhole will remain incomplete. It is time she awakened...But first, you must give the princess back her heart...Don’t you see yet? The princess’s heart is responding. It has been there all along. Kairi’s heart rests within you!...So, I shall release you now, Princess. Complete the Keyhole with your power. Open the door, and lead me into everlasting darkness!"
- Famitsu Interview; Interviewer: "Is Anti Form originated from when Sora got turned into a Heartless?" / Nomura: "Story-wise, yes. Drives are very strong so Anti Form exists as a side-effect of using too much power. System-wise, I wanted to create something strong but troublesome."