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Replicas (レプリカ) are artificial copies of people. Created by Organization XIII, they are a special type of Nobody that first appears in Kingdom Hearts Chain of Memories.

Kingdom Hearts Chain of Memories
The Replica Naminé is forced by Marluxia to tamper with Sora's memories in an attempt to gain the Keyblade's power. Riku Replica also absorbs Zexion to gain his power.

Kingdom Hearts 358/2 Days
The Organization uses the Replica Xion to collect hearts. This is in accordance with its plan to create Kingdom Hearts, which will enable its members to become whole again. The Organization also searches for the Chamber of Waking and develops the Replica Program.

Kingdom Hearts III
The Organization creates replicas that are nearly indistinguishable from real people.

Nature
Replicas are blank figures, akin to mannequins; they take the form of whatever heart or data of a heart is placed inside of it. They have the same skills as the individuals used to create them. They also have the unique ability to absorb other individuals. The Riku Replica was created using data Vexen collected from fighting Riku, while Xemnas used Sora's memories to make Xion, an imperfect replica of Roxas. In the manga, Vexen fashioned 44 replicas of himself.

Over time replicas like the Riku Replica and Xion formed identities, and therefore "hearts", of their own, becoming their own people.

In Kingdom Hearts III, the real Organization XIII re-recruits Vexen to continue the Replica Program. With his research, they are able to create replica bodies for the hearts of the time-traveling members of the Real Organization, allowing them to stay anchored in the present time. A new Riku replica, Dark Riku, is also created. Wishing to atone, Vexen has Demyx deliver a blank replica to Ienzo in order to provide Roxas with a body separate from Sora. Likewise, once Dark Riku's heart is forced out of his vessel, the now blank replica is used to give Naminé a body.

In Scala ad Caelum, twelve replicas modeled after the members of the real Organization XIII fought Sora, Donald, and Goofy before Xehanort himself confronted the trio.

As Vexen explains, these newer replicas become real people once given hearts, unlike previous models which were more akin to Nobodies.