During his school years, he was in a small gang of high school kids. When his gang members came back with Ren, he was not pleased with him. One of his gang members started to empty out Ren's school bag when his deck box came out. When Tetsu asked what it was, Ren explained as a card game called Cardfight!! Vanguard, a popular game that is taking the world by storm currently. Tetsu was looking at the cards when Ren noticed that the cards were glowing a bright rainbow like color around them. Ren ran to him and told him to play the game. At first, Tetsu would turn down Ren's offers to play the game whenever he came to the hideout. Evenually, he gave in and took Ren's offer of playing the game. They fought on the rooftop of the school. When Tetsu lost, he asked Ren to battle him again until he finally wins in the end. Tetsu wanted to battle more, but Ren told him that it was getting late. That was when Tetsu noticed that they spend the whole day playing this game and he decided that this game is worth playing.
He is the more quiet member of his team, only talking when he needs to. Although he is tall and appears to be intimating, he is actually a gentle and kind person. He cares very much for his teammates and always help them when can. When it comes to children, he would be shown answering their questions about Cardfight and doesn't mind helping them out with their decks. Like one time he offered some opinions on Emi's choice of Grade 3s for her deck while commenting on Kamui's play style. When cardfighting though, he can be absolutely relentless and will call his opponents out on their weakness if they are not up to the level they should be.
One of the founding members of the Foo Fighters. Tetsu acts as Ren's right hand, and is known as the "General" of Team Asteroid. He uses a Dark Irregulars deck, which fills up its soul with many units and to use as ammunition against opponents.
Tetsu was the first person to have PSY Qualia used on him, in an accident that gave him a lasting impression of Ren's strength. Even then his ties to Ren were strong, and he felt obligated to look out for his friend; after Kai's desertion, Tetsu chose to stay by Ren's side. Presently he acts as second in command.
In Episode 62, after Asaka lost and was fired from Team Asteroid. Tetsu told her that he would like to see Ren lose and that Aichi might be the one who can defeat Ren, not Kai.
During the second season, Tetsu plays a more of a supportive role in the new Asteroid team giving his spot in the lineup to Kai and briefly lending his deck to Ren during his fight with Kenji. He is frequently challenged by Kamui only to beat him each time. During their last confrontation in the season, Kamui finally defeats Tetsu (though he was in disguise so Kamui did not know he defeated him).
Tetsu, who recently graduated from Fukuhara High School, becomes the coach of his high school's cardfight club. Tetsu also invited Team Dreadnought to his school for training matches. During the VF High School Championship preliminary rounds, Tetsu watches over his club and their fights.
After the VF High School Championship preliminary rounds, Tetsu is challenged to a match by a Reversed Kai. He loses to Kai which results in him becoming Reversed and then going on to fight with Team Brilliant Stars which in turn also Reverses them. Tetsu waits on the rooftop of the Foo Fighter building for Ren to challenge him. Both of them soon began the cardfight. They reminsce the times where Ren bugs him into playing Vanguard which Tetsu gives in after many tries from Ren. Tetsu found Vanguard enjoyable after a match and decides to have a rematch until he beats Ren. After a number of fights, Tetsu finally defeats Ren and calls Ren a Foo Fighter.
Crawl out of the Gates of Hell... Bring forth the power of evil! I ride Demon World Marquis, Amon!
Crawl out of the gates of hell and wield that evil power! I ride... Demon World Marquis, Amon!
Spread the wings of your soul, and unleash that noble power of darkness! Break ride! Demon World Marquis, Amon!
Wicked wings, flip in the darkness! I ride... Stil Vampir!
Sinful king praying for dark proseperity... king who has embraced the despair of light in those wings... The time has come for you to awaken! I ride... King of Diptera, Beelzebub!
From the depths of a cavern where even light cannot reach delivers the ultimate despair I ride Dark Lord of Abyss!
Spirit king of underworld, come and scatter death in this world! I ride No Life King Death Anchor!
Zero spirit in your soul. Shall you come out and bring out the dead spirit into your soul! No Life King, Death Anchor!
Demon of demons, god that rules the demon world! Appear before me, and turn this whole world to hell! I ride... Demon Marquis, Amon "Reverse"!
Tetsu's signature clan is the Dark Irregulars, a clan which focuses on heavy soul charging in order to gain advantages. Each iteration of the deck focuses on different usage of the soul such as for Mega Blasting for Stil Vampir, Gaining power for the Amons, and Criticals for Blade Wing Reijy.