Kingdom Hearts Birth by Sleep
While Aqua was wandering in the Realm of Darkness, a Darkside rose in front of her. Seconds later, a large army of Darksides appeared around her as well. Aqua summons the Master Keeper Keyblade to confront the Pureblood Heartlesses. But as the Darksides were slowly approaching her, Aqua began to lose hope and decides to let the darkness consume her. As one of the Darksides was preparing to finish her off, two shimmering lights appeared out of nowhere, and destroy all of the Darksides in a matter of seconds. It was revealed that both Terra and Ventus's Keyblades, the Earthshaker and the Wayward Wind saved her. Aqua then looks on as the Keyblades disappear from sight.
As Sora was processing through the Awakening, he saw his shadow growing into a Darkside. The Darkside attacked Sora, but Sora retaliated by attacking the giant Heartless. The two fought until the Darkside was eventually sucked into its' darkness.
The Darkside later attacked Destiny Islands after the Door to Darkness was opened. Sora and the Darkside fought each other until the Darkside was sucked up into a dark cloud that loomed above the islands. Despite defeating the Darkside, Sora was also sucked up along with it and ended up being transported to Traverse Town.
While Sora, Donald and Goofy were at the End of the World, another Darkside rose up from the ground below and interfered with the battle between Sora and Ansem. Despite the Darkside's best efforts, Sora managed to defeat the giant Pureblood Heartless.
Later on, after Sora and his friends managed to defeat Ansem and the World of Chaos, the trio got a glimpse of a large number of Darksides within the Door to Darkness. To prevent the horde of Darksides from exiting the Door of Darkness, Sora, Donald, and Goofy attempted to close the door. Despite their best effort, they could not get the door to budge. But just when the Darksides were about to exit the doorway, they were all quickly destroyed by King Mickey.
Kingdom Hearts 358/2 Days
While Roxas and Xion were in Twilight Town, a Darkside appeared out nowhere and attacked the two. After Roxas managed to weaken the Darkside, he found himself strangely weakened, and his Keyblade was knocked out of his grasp. The Keyblade reappeared in Xion's hands. With the Keyblade in her hand, Xion managed to destroy the massive Heartless in a single strike.
Kingdom Hearts: Chain of Memories
While Sora was in Castle Oblivion he eventually encounters a Darkside in the card world of Destiny Islands, with the Heartless appearing in a way very similar to his first encounter with a Darkside. And much like Sora's first encounter with the Darkside, Sora managed to defeat the Darkside as well.
Kingdom Hearts II
A Darkside makes a cameo in Kingdom Hearts II. It appears in Roxas's dreams of Sora's memories.
Kingdom Hearts coded
A Darkside appeared as a data version of itself. It confronted Data-Sora, but in the end, Data-Sora managed to defeated the Data Heartless.
Kingdom Hearts: Dream Drop Distance
Darkside also appeared as a cameo in Dream Drop Distance. While Riku was watching all of Sora's memories so that he can dive into his best friend's heart, he saw the memory of Sora facing off against the Darkside that appeared on Destiny Islands.
Kingdom Hearts III
During Sora's Dive to the Heart, he passes into the edges of the Final World and encounters a semi-transparent Darkside.
Powers and Abilities
Darkside is a powerful Heartless. It possesses great physical strength, and endurance. It can generate darkness in various ways such as creating dark energy orbs, and creating a pool of pure dark energy. It also can summon an army of Shadows from the pool of darkness.
- Darkside is, so far, the only Heartless to appear in every Kingdom Hearts game.
- The Darkside's name is derived from the science-fiction horror serial of the same name.
- It's possible the Darkside Heartless that constantly attacked Sora in the first game is a reflection of Sora's inner darkness. Evidence supporting this includes Sora's shadow turning into Darkside in Sora's Awakening, and Sora's Heartless transforming into Darkside after collecting enough data in Re:Coded. Also, the Darkside Heartless is currently the only Heartless to appear in every game in the series so far, possibly alluding to the fact that Darkness is never far from the Light. In this case, Sora is Light, and Darkside is Darkness.
- It's also hinted that the Darkside that attacked Roxas & Xion was the one that constantly harrassed Sora (which is highly possible since because Roxas is Sora's Nobody and Xion is Sora's clone, Darkside may have known that they were, in a way, Sora).
- The Darkside that constantly attacks Sora is also noticeably larger than all the other Pureblood Heartless, including other Darksides, possibly as a way for people to know it's Sora's Darkside.
- It is also believed that Sora's Darkside might even be the absolute king of the Pureblood Heartless.
- Numerous of guides for the Kingdom Hearts games also refer to Sora's Darkside as Sora's nemesis.