Darkside

The Darkside is a Heartless who can be found as a boss in all games in the Kingdom Hearts series.

Kingdom Hearts
''Enormous Heartless that appeared in Destiny Islands with Shadows. It led the Shadows in the battle against Sora when he first received the Keyblade. Although Sora defeated it, the island was swallowed by the darkness.''

Kingdom Hearts: Chain of Memories
''The Heartless that attacked Sora's home, Destiny Islands.
 * Sora's Story

''Sora fought valiantly to protect the islands and his memories there.

''The Heartless Riku attracted when he was seduced by darkness. It destroyed Destiny Islands.
 * Reverse/Rebirth

''Zexion called Darkside in order to subdue Riku.

Kingdom Hearts 358/2 Days
''This giant Heartless will devour all around it if left unchecked.

''The dark homing blasts fired from its chest can be turned back on it with a block. Target its hands or, if possible, its head.

''TIP: Earth-based attacks like its homing blasts can halve your current HP.

Kingdom Hearts coded
''A giant, heavy-hitting Heartless that tried to put Sora's lights out. And while Sora triumphed over the Darkside, he wasn't ready for the downside: getting swept into the dark torrent that ensued.''

Kingdom Hearts Birth by Sleep
In the ending of Kingdom Hearts Birth by Sleep, Aqua is seen wandering in the Realm of Darkness. A Darkside rises before her, several more appearing around her as well. Aqua summons Master Eraqus's Keyblade, but loses hope and decides to let the darkness consume her. As one of the Darksides prepare to attack, two shimmering lights destroy them. Closer examination shows that the lights are Terra and Ventus's Keyblades, Earthshaker and Wayward Wind. Aqua then looks on as the Keyblades disappear from sight.

Kingdom Hearts
As Sora progressed through the Awakening, his shadow eventually grew into the Darkside. Sora attacks the giant, but is eventually sucked into its darkness. The Heartless later attacked the Destiny Islands after the Door to Darkness was opened, eventually being sucked up into a dark cloud that loomed above the islands, whereas Sora was sucked up along with it and transported to Traverse Town.

A Darkside later rose up from the ground below and interfered with the battle between Sora and Xehanort's Heartless at End of the World. Sora also glimpsed a number of Darksides within the Door to Darkness before they were destroyed by King Mickey.

Kingdom Hearts 358/2 Days
The Darkside is fought in Twilight Town (during a mission involving Roxas and Xion) in Kingdom Hearts 358/2 Days. After Roxas depleted its HP, he found himself strangely weakened, his Keyblade knocked from his grasp. The blade reappeared in Xion's hands, and then she destroyed the massive Heartless in a single strike.

Kingdom Hearts: Chain of Memories
In Kingdom Hearts: Chain of Memories, Sora fights the Darkside in the card world Destiny Islands, with the Heartless appearing in a way that is very similar to how it appeared in the first Kingdom Hearts.

When Riku enters the card world, he too faces Darkside when Zexion creates a replica of Riku and transforms it into the Darkside to show Riku what he "truly was."

Kingdom Hearts II
In Kingdom Hearts II, a Darkside appears in Roxas's dreams of Sora's memories.

Kingdom Hearts coded
A data version of the Darkside appears in Kingdom Hearts coded and it is battled by Data-Sora.

Appearance
A Darkside is a massive, muscular, humanoid Heartless with black skin. Its legs are rather short and its feet are very thin and curl upward. Its arms are quite long, and it has two relatively small, twisted wings on its back. There is a large, heart-shaped hole in the Heartless’s abdominal area, which penetrates straight through from the front to the back. Its head is covered in dozens of twisted, black tentacles. Only its small, glowing yellow eyes are visible.

In Kingdom Hearts, Darkside has a flap of skin where the heart on his body creases at the top, whereas in Kingdom Hearts Re:Chain of Memories this flap of skin was removed for unknown reasons.

Origin
The Darkside's name possibly refers to the fact that it sides with the darkness. It could also refer to the first Kingdom Hearts game, where it made its first appearance and how it was spawned out from Sora's shadow in his Dive to the Heart.

Trivia

 * Darkside holds a number of distinctions among bosses - It is the first boss to be fought in the series, the first boss to be fought twice in the series, and the first boss to be fought twice in a row in the series.
 * The Darkside battle shares many similarities with the Twilight Thorn, a gigantic Nobody. Both appear in Roxas and Sora's Awakenings, respectively, and are the first bosses of the game. Their Japanese names, Twilight Zone and Darkside, are both science-fiction horror serials.
 * The Darkside is the only Heartless boss to appear in every game. However, in Kingdom Hearts Birth by Sleep and Kingdom Hearts II, it is only seen in cutscenes.

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