Impersonator of Death

One of Pegasus' hired Player Killers. He replaces the Ventriloquist of the Dead, from the manga.

He claimed that Kaiba was dead and that he was Kaiba's ghost back for revenge. He took on the form of Seto Kaiba to duel Yugi Mutou, and also used Kaiba's Deck in the Duel. However, once the real Kaiba is revealed to be alive, the Mimic was forced to reveal his true form, which was an ugly, obese clown.

In the Japanese version, the Mimic was either an extremely talented master of desguise or more likely a shapeshifter, taking into account that he seemed capable of altering his own body weight. in the English dub, the Mimic claimed to be Kaiba's evil side which Dark Yugi had seperated from him and banished to the Shadow Realm, and that he was brought back as a seperate being by Pegasus with the power of the Millenium Eye.

(Source: Yu-Gi-Oh! Wiki)