It was all planned so the eldest sister could use him... I mean, how convenient for her to find someone whom she can use however she wants, who had their spirit broken and is desperately looking for a hero.... Especially a guy as good looking as him >.> I can imagine her using Bai Yu to assasin Yan An and/or the second sister to have less competitor for the throne... Or even better, make him get closer to the Empress and order him to kill her... Since the second sister is out of the picture for a bit, she can take the opportunity to get her own mother killed and usurp the throne... Make very convenient excuses as the second sister is too temperamental while her youngest sister is unreliable and all that sh*t... Yep, this sounds about right to me >.>