Owari no Seraph, Kimetsu no Yaiba Recommendations
Very Similar first episodes
Both main characters are fighting to turn someone 'back into a human.'
Humans fighting demons/vampires
Just like Kimetsu no Yaiba, the world-building of Owari no Seraph involves demons, though vampires are the biggest threat in the latter series. The main characters of the anime have a lot in common when it comes to personality. Besides, the main character of Owari no Seraph has a friend who goes through a terrible transformation, which is similar to what happened to Tanjiro and Nezuko in Kimetsu no Yaiba. In short, if you enjoyed Kimetsu no Yaiba there's a good chance you'll enjoy Owari no Seraph too, and the other way around.
Both are shows about a young boy who has his family killed by monsters but one sibling survives and that sibling becomes one of those monsters. The MC then joins an organization comprised mostly of teens that specialize in killing those monsters and they have to use special swords to kill those monsters.
MCs losing a companion and there companion changed into a vampire/demon
MCs having there family killed.
Wanting them to be normal again.