Tanjiro Kamado:
He felt super guilty and terrible after you left. He realized how wrong he was for telling you to quit and to leave him alone when you were only worrying about him.
Zenitsu Agatsuma:
He was really sad. He wanted hugs and cuddles but he felt scared that you'd yell at him again for being clingy. He cried a lot. And I mean it.
Inosuke Hashibira:
He felt really, really, really guilty. He was so mad at himself that he underwent a shiz load of tough training. He also went on a lot of missions in hopes to be able to buy you your favorite thing in the world. uWu
Giyu Tomioka:
He really did feel bad. He was just preparing your first birthday surprise with a few other pillars and with your father. He didn't mean for it to leave you unloved, unwanted, or ignored. He missed you so much.
Sabito:
He cried. Silent sobs were heard in the room he was in. He knew it was his fault that you'd ended up at the Butterfly Estate going through a whole week recovery. If only he'd been logical, he could have gotten revenge for you. He never, ever forgave himself, he missed you terribly. Poor Sabito thought he wasn't good enough for you after he let you get terribly wounded. He cried himself to sleep almost every night. T°T
Yushiro:
He understood why you didn't want to see him. He thought he'd explain everything once you were able to see him without breaking down. He missed a lot though. Sometimes, he would sit in a corner to think and reflect for a day straight.
Muzan Kibutsuji:
He noticed that you purposely avoided him and that you absolutely refused to talk to him. He was angry at first. Then he realized that the day you were supposed to inform him of what had happened with your mission, he heard light footsteps run towards your room after he had finished slaughtering an Upper Moon who had dissatisfied him. He felt guilty for scaring you and for getting angry at you when it was his fault.