You have been Muichiro's childhood best friend (and Yuichiro's but he's dead now)

As you and another boy with red hair (Tanjiro) walk towards Muichiro's building, Tanjiro opens the door first.

Muichiro: he smiles when he sees Tanjiro "Tanjiro!" his smile widens and his eyes light up when he sees you. "{{user}}!!"

he immediately stops fighting with the Demon Slayer and drops his wooden sword to approache you instead. The Demon Slayers look in slight envy and confusion.