Giyu Tomioka has been quiet since his debut in season 1 of Demon Slayer, but lest we forget the introverted Hashira who spared Nezuko's life and was willing to put his life in his faith in the Kamado siblings over his belief that Nezuko will not harm humans.
An enigmatic character, Tomioka is a man of great skill and reflexes. He even has some of the anime's best humerous moments, even if they are at his expense. While he is a humble character, his haori is quite striking, for it is two different patterns. Manga readers know that there is an unfortunate reason for this.
Who is Giyu Tomioka?
Tomioka is one-ninth of the Hashira, the very best fighters of the Demon Slayer Corps. To become a Hashira, members of the Corps must kill 50 demons minimum or defeat a member of the 12 Kizuki, the 12 most dangerous demons selected by Muzan Kibutsuji. Tomioka is the Water Hashira, given that he is trained in Water Breathing. As a person, he is stoic and quiet; which is a common butt of jokes amongst the other Hashira.
He does not speak much and tends to be distant from the rest of the Hashira (save for Shinobu, who tries her hardest to befriend him in her own awkward way). He does, however, have a solid sense of justice and will defend anyone who is abiding by it, such as Nezuko Kamado. In his past, Tomioka has witnessed the losses of two people very close to him, and he immortalizes them in his haori, namely his sister Tsutako and friend Sabito. Half of their haoris make up Tomioka's whole one, a touching and sad tribute.
Tsutako Tomioka, The Left Half
Tomioka had an older sister named Tsutako; while not a lot is known about her life, it tragically yet heroically ended when Tomioka was younger. The day before her wedding, in an act of selflessness and love for her brother, Tsutako hid a young Tomioka while demons attacked and killed her. She protected him and guaranteed a future for him through her actions. Her haori was maroon, the left half of Tomioka's haori.
Sabito, the Right Half
We first see Sabito in season 1 of Demon Slayer when Tanjiro begins training with Sakonji Urokodaki in the art of Water Breathing. He is a thirteen-year-old boy with rare pale hair and a scar across his left cheek. Sabito played a substantial role in Tanjiro's training, for he was a tough opponent in fights against Tanjiro during practices. He is a serious and blunt person by nature, as is reflected in his training techniques with Tanjiro. Manga readers know that Sabito and Tomioka actually had a very close bond.
When Tomioka began his training with Urokodaki, he became friends with Sabito, who was also a trainee under his tutelage. As time grew, the two boys became very close. They trained hard together and knew each other very well. They are both serious personalities and have strong senses of justice. Still wallowing in guilt after Tsutako's sacrifice of her life for his, it was Sabito and his bluntness that brought Tomioka back to reality. Ever since that terrible day, Tomioka has been wrestling with the cost of his mortality. He feels like he should not be alive and would be better off dead for the sake of his sister's life. This angers Sabito, who scolds Tomioka for disrespecting Tsutako's death. He believes Tomioka should not waste the future she gave him with her selflessness, and remember that Tsutatko will always be in his heart with him.
Sabito had the makings of a fine swordsman and leader. He saved many fellow trainees in Final Selection from demons, including Tomioka. The infamous Hand Demon, who had a personal vendetta against Urokodaki's students, was the final demon Sabito would fight. When the boys entered Final Selection, Tomioka exited without Sabito, the only student who did not pass. Sabito sacrificed his own life against the Hand Demon just so Tomioka would survive. This is deja vu for Tomioka, as Sabito's actions mirrored his sister's. In memory of Sabito, he joins the patchwork of Tomioka's haori, the bright geometric right half.
A symbol of those he loves, Giyu Tomioka has a permanent reminder of his duty as a Demon Slayer, Hashira and a person. His first-hand witnessing of ultimate sacrifices not once, but twice, in his life has built a desire in him to emulate the same selflessness. This is evident in his willingness to commit seppuku if Nezuko kills a human, which is against his vouching for her. With the memory of Tsutako and Sabito that he literally wears on his sleeve, while it can be a painful reminder of the past, it is also a blueprint of he wants to be.