Yuuki Tenpouin is a Code: Breaker whose codename is Code: 03. He has the ability to manipulate sound. His addition to the Code of Hammurabi has yet to be revealed.
The current Code:03. At first glance, he seems pretty calm and childish but goes on a rampage if anyone brushes him. When he's first introduced he hasn't eaten in three days, forgot where he lived, lost his wallet and gave his phone to a stranger and spent the rest of his money on a crane game. Oogami describes him as the "most barbaric code breaker". He speaks using Kansai-ben. A major love of his is the stuffed animal named Nyanmaru. Despite his violent tendencies, he genuinely rewards kindness, and apologizes by giving gifts to people - particularly Nyanmaru merchandise. In chapter 35, he gives Sakura a stuffed animal for her genuine concern for his well-being, and in Chapter 36, he attempts to get a golden Nyanmaru toy for Toki as compensation for beating him up, even though he didn't want it. In chapter 36, he admits that he hatesEden, but has joined the organization so that he can take it down. In recent, he has mistaken Sakura as the true identity of Nyanmaru when her saying reminds him of the animated character, as such he calls her Nyanmaru instead of her name. He hates Heike and unlike the others he doesn't fear him, going so far as to pour tea on his head. He seems to have suffered a traumatic past, resulting in him becoming kind rather than evil.
He sleeps in a mansion and is referred to as President by the servants. He is actual very intelligent, graduating at the age of 12 but only because he hated studying. He runs a successful corporation that focuses around his childish ideas (later based on his friends) to create cartoon characters. However, all the ones he made, he believes they aren't nearly as good as Nyanmaru. He is currently staying at the Student Council President's house after his house was completely destroyed.
When he was a child he destroyed things by accident just by speaking since he couldn't control his power. Makoto who was his friend made a Morse code for him.
Yuuki joined the hunt for Ogami's left arm, In chapter 94 the reason Yuuki joined the hunt for Ogami's left arm is to get medication for makoto that eden advanced medical staff offers.
In Chapter 98, He attacks Ogami at his school, even attacking Sakurakouji. Saying he wants to save his friend (Makato) And he doesn't care how many lives he has to sacrfice to do so. He seems the most determined to take down Oogami, just to save his friend Makato, these probably shows that he really cares alot for Makoto.
Yuuki greatly admires Oogami, finding him very kind. In chapter 101, it reveals the past of Yuuki and Oogami. Yuuki(in lost form) and some other cats was protected by Oogami in an explosion. It was the first time they met. He said that ever since he met Oogami, his heart was heart was shaken over and over by him. He also admits that Oogami is an idoit that he seriously loves. However, he is still determined to save Makato. In chapter 102, Yuukki manages to rip his arm off, killing Oogami almost instantly.
Those out the whole series, Yuuki never accepted anyone else but Makato and Shigure as his friends, but in chapter 106, he accepted Oogami and Sakura as his friends. In chapter 108, Yuuki got badly injured by Heike's attack, when he protected Sakura's classmates from the attacks. In Chapter 113, he is currently training with the President. In 118, he appeared in front of Toki to protect Ogami. He tried to convince Toki not to go after Ogami (he even try to give him Nyamaru). However, Toki still insist in doing it and tells him if he gets in his way he will kill him.
Like Heike, he too seems to not prefer the idea of killing, as he went out of his way to save a pregant woman and a small child from a explosion. When angered his skin turns red. And when he turns into his lost form, a small black cat, he still retains his rage and his ability to speak. Even going so far to attack anyone who makes fun of him in that form. He is also known to be very bad mouthed in his lost form. This form could also explain why he is so fond of the anime character Nyanmaru. It has been said that the way Yuuki Acts isn't is "True Self."
In chapter 99, Yuuki was desperate to retrieve Oogami's arm, in order to save Makato, attacked the innocents (Sakura's classmates). 'He'll attack both friends and enemy alike..In other words, Yukki is the most barbaric code:breaker.' mentioned by Oogami in chapter 35.It could be because it involves the necklace he wears around his neck. It is hinted in Chapter 98 by Toki that Yuuki's idea of taking off that necklace is taking things to the extremes.In chapter 104, it is revealed that Yuuki's necklace actually contained pills, comfirming what Toki said in chapter 98 that it was risk when Yuuki started to struggle in pain after he ate it.
In chapter 105, a flashback of Yukki shows that he was about to help some injured people in an accident (cause unknown). Yuuki was persistent on saving those people despite resistance from Shigure and Makato. He used his sound powers to help people. However, a huge chunk of debris almost crushed him, only to be saved by Makato. This caused Yuuki to stop making friends and spending his time with his powers to save Makato, saying that is his resolve for being a Code:Breaker.
In this chapter it also showed that the medicine Yuuki ate forcefully amplies his powers,and he cannot lose his powers. Yuuki tried to give all of his powers to Oogami, saying Oogami was the only one able to help save Makato and everyone else. During this process, Yuuki released a huge sound wave, alerting Toki, Heike, Shigure and even Makato. He almost killed himself, only to be stopped by Sakura' hug (her rare kind powers).
At the end, Yuuki heard Morse Code from Makato himself, telling him that he realised Yuuki is really a kind-hearted person and hoped Yuuki would bring more friends to visit him (Makato) next time.