After losing his sister Tsutako Tomioka and shortly after losing his best friend Sabito, he became cold and rarely smiles. While fundamentally kind and righteous as his name suggests, his personality turned taciturn and stoic after Sabito's death. He does not easily sway or show emotion, maintaining his characteristic dead eyes and expressionless face in almost any situation.

He used to look after orphaned children at a temple, but one of the children lured a demon to the temple, leading to the death of all the children except one. He was nearly executed after being arrested for murder, but his life was saved by Ubuyashiki Kagaya.

The source of Obanai's trauma. When Obanai escaped from his prison at age 12, fifty members of his clan—everyone except himself and a female cousin—were slaughtered by the Serpent Demon. His cousin shamelessly blamed him, shouting, "If you had just been a sacrifice, the family wouldn't have died!"

He is constantly suffering internally, unable to shake off the sins he committed in his past. He frequently says that he is destined to fall into hell.

His twin brother. They were close when their parents were alive, but after their parents passed away and left the two of them alone, they grew distant. However, Yuichiro revealed his true feelings just before he died from a demon attack.

After losing his mother Ruka to illness, his father Shinjuro turned to constant drinking, leaving Kyojuro without a single word of praise.

His current sharp and aggressive personality stems from a tragic past where he had to kill his own mother with his own hands after she turned into a demon and killed his younger siblings. Driven by hatred from this trauma, he hunts demons relentlessly, which has soured his temperament.

After her parents were brutally murdered and she lost her sister—the only person she could lean on and her greatest reason for living—Shinobu lives only for revenge against Doma, the one who took her sister away, with no intention of living for anything beyond that goal.

She joined the Demon Slayer Corps to express her true self and find someone who would love her for who she is. Since she is attracted to strong men who can protect her, she joined the Corps where many strong individuals gather.