I wouldn't necessarily count Katie as a harem member, more along the line of friends with benefits. While she has shown some romantic interest towards the MC, due to his relationship to Emily, it appears, she keeps it to just that. She doesn't really have the devotion sexually that the chosen have. Remember, even though he has offered the chosen to have sexual interest outside of their pact, well except for Molly, they refuse and devote themselves to the MC in a marital way. Katie doesn't and even admits to not be able to be such due to her being an Evoker. The closest to a 4th member of a harem would be Jenny, but theirs appears to be more of a dom/sub fling than an actual devoted harem member.
Harem in reference to kingdoms requires a devotion aspect, whether consented or enslaved, not just sex. Else even Chloe, possibly, would be considered a member.
As to lovers and side flings, with exception to Jenny, and possible Chloe, both of which Emily doesn't appear knowledgeable of, she pretty much just gave the OK to companions, meaning Kitsune and Katie on top of the Chosen, at this point. If I remember right, I think she even made a defining joke about not having the women of Camelot into his harem.