Wait a minute... I think I just realized something. Heh, that's clever. So originally the title "Blood Princess and Knight" referred to Bai Ji and Ji Bai, the two warring sides of the protagonist, who had different values and goals.
But then the two of them became One. A lot of people seem to still be confused about that, but it seems pretty obvious to me that the fragmented psyche finally snapped back together, and it resulted in the form and personality of Bai Ji being mostly dominant, because that is ultimately who she was, who she became when she was reborn as a vampire, complete with a second childhood.
The Ji Bai personality being a bit of self-delusion, a claim made in denial that they're still the human they used to be. But once the honorable and empathetic knight rejoined the bratty princess, not only has she physically become more adult, but has displayed more emotional maturity and a strong code of ethics and morals, indicative of the sides of herself that she shaved away and personified seperately to maintain this sharp dissociative divide.
But this does present a problem of sorts, the title is no longer accurate if there is not a Blood Princess AND a Knight, and that's where the deal with the spirit of St Lun came in, to inherit her power as a holy knight, and eventually realize a knightess form in her image... but this too passed.
Once the St Lun Inheritor form died, some of the lingering questions about it became moot... but in this newest arc we're seeing a knightess with an extremely similar design to St Lun that Bai Ji is now teaming up with, a blood princess and a knight again, fighting side by side, since this mystery individual pretty clearly has some kind of ties to St Lun as well, it seems likely that some of the loose threads from that particular plotline will be picked up again now.
So in short, people reading this series who are angy about Ji Bai no longer being in play? Wanting "him" to come back? Far as I can tell, he was always a phantasm, a temporary coping mechanism, a construct of the person the now empress used to be. Might need to accept that the same way she did.