If partnership is loosely defined as two people working together as equals, the range of pairs that the term covers is much greater than you'd expect. In a business or work context, "equals" are fairly easily identified. Perhaps they're two detectives who work together and share a car, or perhaps they're two lawyers who share a controlling interest in the firm. In personal life, "partner" is used almost exclusively to refer to a romantically significant other. But neither of these quite fits the relationship between Vance and his favourite assassin.
Love and hate are sometimes said to be two sides of the same coin - neither can exist without the other. I'm reminded of the way that two soldiers who fought on opposite sides of the same war can have a certain understanding that no civilian could ever achieve with either of them. Vance and Kai spent years fighting the same battle, even if it was against each other, and in a way they came to love each other. In fact, Kai's feelings for Vance were such that he was the only one she felt she could trust to end her life. As it turned out, his feelings for HER were such that he couldn't, despite her threats. It was Jackie Vance who took it upon herself to end the life of the woman who was, in a way, her husband's mistress.
I think the Vances might need some marriage counselling after this.
