Ghost Kaiba, known in the Japanese version as the Imitator of Death and in the English video games as the Mimic of Doom, was one of Pegasus' hired Eliminators. He replaces the Ventriloquist of the Dead, from the manga.
In the Japanese version, he was either an extremely talented master of disguise or, more likely, a shapeshifter. He even seemed capable of altering his own body weight. His true form was that of an ugly, obese clown who dressed up as Seto Kaiba to Duel Yugi Muto.
He claimed that Kaiba was dead and that he was Kaiba's ghost back for revenge. In the dub, he said he was Kaiba's evil half that Yugi had sent to the Shadow Realm. The Imitator had taken Kaiba's Deck and used it in the Duel.