In tsubasa RC Fai was telling Yuuko that he liked making fun of Kurotan {I dont know his real name} but he?

then said that he like teasing him so much beacuse of all the faces he made that he didnt realize that he had "crossed the line" what line was he talking about?? Did he mena he had gotten to friendly with the person cause Kurotan was always mad so I dont think he meant that he went to far with the teasing. Im pretty sure it was after he turned into a vampier and called him by his real name Kurogane {thats it right?} for the first time. So like in chapter a hundered and something {146?? maybe-doubt it}