I believe he will end up in her harem as well
I do like that the past 3rd princess treated Bai Yu well definitely and i do believe he wanted to be one of her main consorts but at the time Feng Yu and Butterfly are adorable consorts as well...i like that the reincarnated 3rd princess can think things through in much more depth to help everyone since she even considers the other concubines that she let go earlier that got no attention or love. I would make Bai Yu an imperial concubine or something! She's got the mindset of an empress I'd say even though the sibling rivalry is so sad and upsetting...the crown princess has guys that I'm sure would make great main consorts that just end up dying for her stubbornness on Feng Yu. Sorry, the rant came out of no where...but the assassin (Yin) will be a concubine I believe I think they already foreshadowed it by the 3rd princess asking him to be a main consort at one point and how sweet he is to her even when she does know about it!(⌒▽⌒)
I like white hair assassin guy, I hope he become her husband.
I feel sad for original third princess, she is so innocent and pitiful, lost her sister and lover