He is thea character from the anime Demon Slayer and one of the swordsmen of the demon slayers. He is a character from the anime Demon Slayer and one of the swordsmen of the demon slayers.

Using "the" will be understandable, but "a" is more clear. Saying "the" implies there is only one character, but there are multiple characters from Demon Slayer.

He is the character from the anime Demon Slayer and one of the swordsmen of the demon slayers. He is the character from the anime Demon Slayer and one of the swordsmen of the demon slayers.

or: "[...] one of the swordsmen serving as demon slayers."

He is thea character from the anime Demon Slayer and one of the swordsmen ofin the dDemon sSlayer Corps. He is a character from the anime Demon Slayer and one of the swordsmen in the Demon Slayer Corps.

“in the Demon Slayer Corps” is closer to the official phrasing.

Since he looks intimidating and never smiles, no one really talks to him. Since he looks intimidating and never smiles, no one really talks to him.

Since he looks intimidating and never smiles, no one really talks to him. Since he looks intimidating and never smiles, no one really talks to him.

You could also say "because" instead of "since", but both will be understandable.

Since he looks intimidating and never smiles, no one really talks to him. Since he looks intimidating and never smiles, no one really talks to him.

Since he looks intimidating and never smiles, no one really talks to him. Since he looks intimidating and never smiles, no one really talks to him.

Giyu Tomioka Ffrom Demon Slayer Giyu Tomioka from Demon Slayer

Since then, he has been dwelling on his death, and still can't get over it. Since then, he has been dwelling on his death, and still can't get over it.

He used to behave a best friends with named Sabito, and they would train hard together to become swordsmen. He used to have a best friend named Sabito, and they would train hard together to become swordsmen.

"He used to have a best friend named Sabito" sounds closer to what you wrote in Japanese. "He used to be best friends with Sabito" sounds unnatural here because the reader doesn't yet know who Sabito is, so I think "a best friend named Sabito" is a better way to introduce him in your post.

Both Sabito and Giyu were promising apprentice.s. Both Sabito and Giyu were promising apprentices.

Both Sabito and Giyu were promising apprentices. Both Sabito and Giyu were promising apprentices.

Add an "s" to the end of "apprentices" to make it plural. There are two apprentices, so it's plural.

Both Sabito and Giyu were promising apprentices. Both Sabito and Giyu were promising apprentices.

Both Sabito and Giyu were promising apprentices. Both Sabito and Giyu were promising apprentices.

One day, whilen they were fighting against demons, Sabito accidentally died. One day, while they were fighting against demons, Sabito accidentally died.

One day, when they were fighting against demons, Sabito accidentally diwas unexpectedly killed. One day, when they were fighting against demons, Sabito was unexpectedly killed.

I think "accidentally died” sounds unnatural in your sentence, since dying in battle isn’t really an accident. "Accidentally" implies something unintended but not caused by direct combat, like slipping, falling, or a car crash. "Unexpectedly" conveys better that it was sudden and unplanned. “Died” could work, but it’s softer and less direct, so I think "killed” is better because it’s a battle and the demon was responsible.