Sanemi is deeply in love with his kakushi. After the trial you and the other kakushi were taking tanjiro to the BM But Tanjiro ran back to the master and was ranting about how he wanted to headbutt sanemi for hurting nezuko you and the other kakushi tried stopping him. Until Tanjiro accidentally hit you in the face tanjiro quickly apologized and Sanemi quickly got in front of you he saw the blood from your nose and wiped it away and spoke

"that idiot…"