Critias

Critias was a legendary knight from the city of Atlantis, who aided Iroheart in the Battle of Atlantis.

Background
Critias and the other two legendary knights, Timaeus and Hermos mostly resided in the Spirit's World, until Atlanis became corrupt by the Orichalcos. Ironheart summoned them to the human world, to help him defeat Dartz, who was trying to wipe out the population of Atlantis and start fresh. However, Dartz weakened the knights by turning them into dragons.

The three Legendary Dragons had fought alongside rebels from Atlantis and more Duel Spirits, against Dartz, Orichalcos Soldiers and The Great Leviathan.

The battle however ended with neither side being victorious, and Atlantis sunk to the bottom of the sea. The Dragons were sealed in crystal prisons, until they would needed again.

Personality
His personality could be compare to that of Priest Seto or Kaiba himself.

Appearance
Critias, like other legendary knights, is named after one of Plato's dialogues. Which are the source of many of the rumors pertaining to Atlantis.

In his human form, Critias wears dark blue armor. Like the other knights he bears a similar facial expression to his chosen duelist, Seto Kaiba.

In his dragon form, Critias is a dark shade of blue.

Waking the Dragons arc
Seto Kaiba was chosen as a Legendary Duelist to wield Critias. During his Duel with Alister, he was subconsciously taken to the Duel Monsters Spirit World, where he saw Critias trapped in his prison. By removing a sword he set Critias free and got the card "The Fang of Critias", so that he may Summon "Critias" while playing Duel Monsters.

As a card, Critias had the ability to combine itself with Trap Cards. Kaiba used it to make use of his "Crash Card Virus", which Alister had prevented him from using with "Royal Decree". He used it in his second Duel against Alister to Summon "Tyrant Burst Dragon" then 'Blue Eyes Tyrant Dragon".

When Yugi and Kaiba face Dartz, Kaiba combined "The Fang of Critias" with Yugi's "Mirror Force" to Summon "Mirror Force Dragon". Yugi later used "Legend of Heart" to change the dragons back to their true forms. Together with Timaeus and Hermos, Yami Yugi was able to defeat Dartz's "Divine Serpent" and win the Duel.

However, The Great Leviathan was Summoned again. Critias took part in the battle against it, this time his side emerged victorious. He along with Timaeus, Hermos and Dark Magician Girl appeared on the beach to thank Yugi Muto and Yami Yugi before returning to the spirit world.

Timaeus
He gets along with him and doesn't seem to have a rivalry with him.

Hermos
He doesn't seem to have a rivalry with him.

Ironheart'
He follows his orders very well.

Seto Kaiba
He usually helps Seto fight his battles against Alister.

Trivia

 * He doesn't appear in the manga.
 * He doesn't have a rivalry with his two friends.

Voice Actors
all information on Critias came from http://yugioh.wikia.com/wiki/Critias
 * Japanese : Kenjiro Tsuda
 * English : Eric Stuart