He was an illegitimate child born between a mistress and the previous Sasaki heir. After his mother died, he moved into the Sasaki family but was treated harshly.
Having been badmouthed behind his back, he joined the Check it out Gang but later ran away after witnessing their cruel acts and illegal business fearing that the gang would have found out that he was half-brother to a Mimawarigumi commander.
Appearance
Tetsu is chubby and short. He has a round face and has a shave haircut.
B-Boy
As a Bad boy, he dressed up as a New York gangster style along with his Shinsengumi uniform. He has unnecessary accessories such as the sunglasses, cap and gold chains. He also rolled one of his pants legs up to his knee. He tends to be rude to his peers.
C-boy
As a Cherry Boy, his appearance is completely different. His eyes are drawn more like an old shoujo manga.
Sasaki without sunglasses and facial hair.
Personality
When first introduced, he acts like a New York gangster or B-boy (Bad-boy) and he occasionally speaks in rap. He was more selfish as he thinks he can do whatever he wants.
But after Toushiro backed Tetsu when Isaburo bad mouthed him, Tetsu changes into a polite person. Toushiro influence on Tetsu changes so much that Tetsu becomes more hard working.
Relationships
Family
Sasaki Isaburo: His half brother and Tetsu absolutely afraid of him as Isaburo wouldn't hesitate kill him. But deep down Tetsu wants a bond with his brother. Despite the differences, thus when he and Nobume is about to be attacked by a Naraku soldiers while getting Isaburo and their allies escape, Isaburo repaid his half-brother's debt and sacrifice his life to save him and Nobume, so they can escape quickly without Isaburo. With lost of his brother from the enemies' hands, Tetsunosuke and Nobume mourn his brother's sacrifice, as seen in the Boshin War arc of Shinsengumi Rebellion story part.
Friends & Allies
Imai Nobume: Both have a same close common relationship with his brother, Isaburo. As Tetsunosuke finally sees his brother shows his inner kindness sacrificing his life to save him and Nobume, Tetsunosuke, along by their police companions trying to reconsole Nobume's sadness and to honor his half brother's death by a salute.
Tetsu's serve Toushiro as his assistant. At first Toshirou disliked Tetsu because of his Bad boy gangster act, elite background and spoilt behaviour. Toshirou defended Tetsu after Sasaki Isaburo bad mouthing Tetsu in front of him. After Tetsu learnt from Kondo Isao that Toshirou and Tetsu had similiar background and Toshirou knows how Tetsu feels, Tetsu changed from B-Boy (Bad boy) to C-Boy (Cherry Boy). Tetsu decided to become stronger to walk the same path as Shinsengumi. Toshirou mentions in the Baragaki arc that he shamefully learnt one or two things as he watches Tetsu struggles.
