The Imitator of Death, or Ghost Kaiba (4Kids dub), is an extremely talented and demonic master of disguise and an antagonist in the anime Yu-Gi-Oh!.
Pegasus J. Crawford hired this strange figure in to duel Yugi. The Imitator of Death had the appearance of Seto Kaiba, except with dark shadows under his eyes, and his voice sounded strange. He was using the real Kaiba's deck, as it was stolen from him by Pegasus' goons after Kaiba's supposed death. However, Yugi managed to defeat the first Blue-Eyes White Dragon, but the second would have beaten Yugi, had Kaiba not hacked his impostor from his computer. In the end, it was Kaiba using the Heart of the Cards that destroyed the second Blue-Eyes. At this point, it was revealed that Kaiba was alive, and the Imitator revealed his true form, which was a fat and ugly clown.
The Imitator of Death waited patiently until he drew his final Blue-Eyes, but Yugi was able to revive one of the destroyed Blue-Eyes with Resurrection of the Dead and powered it up with his Holy Elf, allowing his Blue-Eyes to destroy the Imitator's and take his life points down to zero, winning the duel. Yugi then used the Millennium Puzzle's power to make the Imitator disappear off the face of the earth.
In the 4Kids version, the Imitator was really Kaiba's ghost or his evil side and was sent to the Shadow Realm instead of Hell. In the manga, he does not even exist in the first place and it is a human ventriloquist who uses a Kaïba-like puppet who challenges Yugi with Kaïba's deck.
Yugioh! The Abridged Series
In LittleKuriboh's parody series Yu-Gi-Oh! The Abridged Series, Ghost Kaiba was a gay clown. No one except Tristan and Kemo believed that he was truly a ghost, and when he revealed himself as a gay clown, he was immediately killed by Yami Yugi with a Mind Crush. Yami apparently doesn't get over the fact that he killed a gay clown, as he references it eleven episodes later when he is dueling Pegasus.
The gay clown also appears in the anniversary of the Abridged Series, in which he is killed offscreen by Yami, much like in the series, to which he says "He's a gay clown. What do you expect?".