Shinazugawa, Genya

Early on in the series, Genya had a short build which stood slightly above Tanjirou Kamado. He had short, messy back hair that what shaved off at the sides. thin eyebrows, and a scar that is covers most of his face's width.

Later on, he reappeared with a wilder look as his height changed significantly, making him taller than most and now has longer hair reached down to his neck. He also gained a large, muscular build along with this as well as various scars all over his body.

Genya is a foul-mouthed and ill-tempered individual with a "lone wolf" character who refuses any help from others and does not show any interest in being polite or having any manners.

This changes later on when after he fought Hantengu, an Upper Moon demon, along with Tanjirou, Kanroji Mitsuri; the Love Pillar, and Nezuko Kamado, and was not as imposing as before.

Genya belonged to a family of nine with a hardworking mother who never slept and deadbeat father who would occasionally beat her despite her weak build, including the children as well.

Some time later, Sanemi had learned that their father had gotten stabbed to death and made a vow with Genya to protect their siblings and mother.

One day, their mother went missing and didn't return almost until daybreak. As suspicion and worry began to cross the minds of the children, alone and unprotected, a loud knocking sound was heard from their door which they presumed to be their mother. It was a demon instead which immediately killed off five of the seven children and scarred Genya.

Leaving no moment for Genya to escape, the demon soon came for Genya. This was interrupted by Sanemi who later went into battle with him telling him to "run for it!!"

Shortly after, Genya went to call a doctor for his injured siblings but soon stumbled across Sanemi who had held the demon at bay and successfully killed it with the help of the sun. He was unprepared though, as the demon who attacked his family was none other than his mother. Greatly confused by what happened, Genya panicked and chose to place the blame on Sanemi and call him a murderer for killing their mother and tricked himself to believe it was a wolf that attacked the family instead.

(Source: Kimetsu no Yaiba Wiki, edited)