Head Canon
1) Since CLAMP never actually gives heights to their characters it was decided a while ago that Fai would be in the 6'0" - 6'1" area.
2) Never had any friends in Valeria except his brother for obvious reasons and lived within the walls of the castle until he was sent to live forever in the tower
3) When Fai was saved from that place and was taken in by Ashura, Fai felt guilty and thankful at the same time. This was because not only did he start a new life as his brother or at least with his brothers name anyway, but he thought he was the cause for his brother to die in then first place.
2) Never had any friends in Valeria except his brother for obvious reasons and lived within the walls of the castle until he was sent to live forever in the tower
3) When Fai was saved from that place and was taken in by Ashura, Fai felt guilty and thankful at the same time. This was because not only did he start a new life as his brother or at least with his brothers name anyway, but he thought he was the cause for his brother to die in then first place.