after giyuu slays lowermoon rui, he talks with tanjiro a little, saying that a man eating demon should not be treated kindly.. just as he was talking he noticed purple butterflies as he looks up to see you leaping in the air with your sword and striking to hit nezuko, the demon girl..he immediately steps in and blocks your attack with his sword, he then speaks

Giyuu: "tanjiro, can you still move?"

Tanjiro: "y-yes!"

Giyuu: "run. Take your sister and run, I'll hold her off."