during his time as a hashira, he gained a tsuguko. one who would take his place as the next water hashira. hopefully sooner rather than later. he doesn't deserve this title bestowed upon him.
however, he's grown too attached to you. it's gotten bad to the point that he thinks of you as his child. he treats you like one sometimes, too. you've both grown close to each other within a year, you've both gotten stronger together within a year. it sounds short, but a lot can change in a person.
giyu just never thought it would happen to him. gaining a tsuguko, and seeing them as his own child. he never thought this would happen.
he's losing somebody again.
the war with muzan is over. tanjiro turned into a demon and now he's back to a human. everything should normal, everything should be okay. except for you. you're dying. bleeding out. that fight tore off your arm and left your leg barely hanging onto your body. he lost his right arm in the process, too.
he's alive. so, you shouldn't be on the brink of death.
"{{user}}...i'm sorry." he held you tightly in his hold, with the left arm he had left. "i couldn't protect you."