Tet is seen wearing a beret with a brim and a yellow clover on top of it. He has a small red-colored heart below his right eye. His most distinctive features are his eyes, for they are multi-colored. Despite the fact of his eyes being multi-color, his right pupil has a diamond shape and his left a spade shape on it. He wears a large metal earring on his left ear which is shown to be a die, a blue choker and bracelets in both hands with the same color. He wears a red hoodie with a chartreuse shirt and typical blue shorts, accentuating the fact that he is only a boy. Tet appears to have all the symbols of a deck of cards on him (the last suit, clubs, is printed in yellow on his hat). He is shown to wear a red sock on his right foot and a red piece of cloth wrapped around his left one.
From his introduction when he transported Shiro and Sora to his world, Tet has been shown to be bright and playful like any human boy of his apparent age. From the rules he set up on Disboard he appears to abhor violence and fighting, probably after seeing what the ancient war did to the world. This shows that he may be somewhat benevolent and peaceful. He also mentions that he usually only spectates, rarely interfering. He dislikes the other Old Deus (possibly not all) and was shown to have a negative opinion of them. He also thinks of them as boring.
A long time ago, Riku, who had a habit of playing chess on his own, always imagined the existence of an 'ultimate gamer' sitting right across the chess board, constantly challenging him to another game with a smile.
During the climax of the Great War, when Riku successfully summoned the Star Cup, he considered himself unworthy of its power and instead prayed for somebody to appear and use the Star Cup to make the world a better place. In his last moments, Riku witnessed the 'ultimate gamer' of his imaginations come to life. The person who stood before him called himself Tet, the final and weakest Old Deus, born from Riku's wish and the overwhelming presence of incredible magic energy. Claiming the Star Cup and becoming the One True God, Tet immediately used his absolute power to repair the damage done to the planet and set forth the Ten Pledges, thus ending the Great War.
After the end of the Great War, as the Exceed races slowly learned to cope with their new lifestyles, Tet would constantly masquerade as a member of a particular race, infiltrate said race and have as much fun as he could. He also enjoys visiting other worlds and parallel dimensions to learn their games and strategies.
6000 years later, during one such trip, he came across a pair of siblings who were said to be unbeatable in any game. Intrigued, he challenged them to a game of online chess and, for the first time in his life, lost. Elated over finally meeting a worthy opponent, Tet transported them to the world of Disboard and presented them with the opportunity to challenge him for the throne of One True God.
Since then, Tet has been constantly monitoring Sora and Shiro with enthusiasm. Not long after Avant Heim joined the Elchean Federation, Tet decided to masquerade as an Imanitian and go play with Sora and Shiro, but ended up fainting on the streets because he hadn't eaten for three days. When Izuna encountered him and fed him some fish, Tet passionately challenged her to various games and recounted the story of Riku and Shuvi to her.
As the One True God and former God of Play, Tet presumably has overwhelming knowledge in all types of games and is very observant to find out cheaters. His abilities are further implied as when Sora and Shiro beat him in a game of chess and they talked in public (light novel) the Imanity were shocked of how their new king beat the One True God in a game of chess. He can also change his appearance at will.
As the One True God, he has a plethora of unknown powers, with the definitive ones being teleportation and levitation. He can also be assumed to possess vast amount of Magical Power, being most likely the being with the strongest affinity for magic in Disboard. Jibril had once stated that even the Old Deus would have trouble summoning a being from another world, and yet Tet has been able to keep two in Disboard for a prolonged period of time with no ill effects.
As of the moment, he does not have any discernible weaknesses. While he has been beaten by Shiro and Sora in a game of online chess, that is the only time when he has been shown to lose, and even then, he pushed them to their limits.
Before gaining the Star Cup, he was known as the weakest of the Old Deus and was only bought to life by the wishes of Riku.
|“||You creatures who have fought with strength, violence, blood and death, who built a tower of corpses that rises to the sky and call yourselves wise, tell me this: What is the difference between you and the dumb beasts? もし強さ、暴力、血と死と戦ったの生き物、 誰が、天に上昇し、自分が賢い呼ぶ死体の塔を建て 私にこれを言う：あなたとダム獣の違いは何ですか？ (Moshi tsuyo-sa, bōryoku, chi to shi to tatakatta no ikimono, Dare ga, ten ni jōshō shi, jibun ga kashikoi yobu shitai no tō o tate Watashi ni kore o iu: Anata to damu-jū no chigai wa nanidesu ka?)||”|
|“||From this piece of Heaven, I command that there will be no plundering or bloodshed.||”|
|—To Disboard, First law stated by Tet (Light Novel Volume 1 chapter 1 part 1)'|
- It is interesting to note that Tet's name is also the 9th letter of the Hebrew alphabet ("ט" in Hebrew). Although it cannot be confirmed at this point, Tet's name ("テト" in Japanese") could be a reference/abbreviation to "Tetragrammaton" ("テトラグラマトン" in Japanese), which would explain why he is the sole god in the series.
- In Volume 6 it is shown that Tet was the imaginary friend of Riku that played with him when he was lonely. He was brought to life through prayer and belief to obtain the Star Grail and put an end to the Great War when Riku was unable to due to "not being worthy" in his deformed, dying body and physically being unable to reach it due to both arms falling off.