Imitator of Death

"It's me, Kaiba, and this time, you don't stand a ghost of a chance!"

Ghost Kaiba is the evil side of Seto Kaiba from the anime Yu-Gi-Oh. However, when Yami Yugi defeated Kaiba in a duel, he gave Kaiba a Mind Crush, which was more directed at his evil side, sending it to the Shadow Realm. The evil side manifested into physical form, often known as Ghost Kaiba or Kaiba's ghost.

Maximillion Pegasus brought him back from the Shadow Realm in Episodes 8-10 to duel Yugi. Ghost Kaiba had the appearance of 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's goons after Kaiba's supposed death. However, Yugi managed to defeat the first Blue Eyes White Dragon, but the second would've beaten Yugi had Kaiba not hacked his impostor from his computer. In the end, it was Kaiba's using the Heart of the Cards that destroyed the second Blue Eyes. At this point, it was revealed that Kaiba was alive, and Ghost Kaiba revealed his true form, which was fat and ugly.

Ghost Kaiba waited patiently until he drew his final Blue Eyes, but Yugi was able to revive one of the destroyed Blue Eyes using Monster Reborn, and powered it up with the Mystical Elf allowing his Blue Eyes to destroy Ghost Kaiba's and take his life points down to zero, winning the duel. Yugi then banished Ghost Kaiba back to the Shadow Realm with a Mind Crush.

In the Japanese version, Ghost Kaiba was not really Kaiba's ghost or his evil side, but rather a clown who wandered through Hell and enjoys imitating people who are dead. Often referred to as the Death Imitator or the Mimic of Doom. He was apparently killed when Yugi Mind Crushed him, as opposed to being sent to the Shadow Realm. In the manga, he doesn't even exist in the first place and it's a human ventriloquist who uses a Kaïba-like puppet who challenges Yugi with Kaïba's deck.

Yugioh Abridged
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?"