As far back as book 2, I've always wanted Mikhail Joan and Natalia Ruyemov to have a shared history together. Starting off first as enemies due to a misunderstanding of who her target was, given that Mikhail looked awfully similar to another tiger of infamous renown, then to family friends and then with the loss of Mikhail's family, it turned to love of a sort. The physical, romantic love wasn't to last due to complications, but there was always a closeness there between the two that I wanted to highlight and expand upon in books 3 and 4. It's a measure of trust that comes from intimacy that only deepened the friendship to where, although they both agreed being platonic going forward was probably the best option, there was always that deep-rooted bond and trust between them that they'd always have each other's back no matter the circumstance. Friends to the end.