A new friend
Yu Dong heard Yu Mai and started pondering about how exactly sue should answer him, she didn't want Yu Mai to develop a master and slave outlook. If he truly began to consider himself as Lang Ru's master it will only hinder their future friendship. And being someone's purchased slave there was nothing great about that either.
As she was someone from the modern world she had no thoughts such as keeping Lang Ru, this young girl who will have countless opportunities at home. She would definitely send her to school and let her do whatever she wanted because that was what was expected of her, as an honest officer and human.
If she told Yu Mai that she bought Lang Ru and her family, it will soon become an obstruction in Lang Ru's future, after all, who would like to have such a past?