The relationship between these two is more similar to the one between siblings.
Yuzuriha is always near Kamui and try to cheer him even if it's not easy and Kamui totally need and care about such sweet hearted girl.
It growed between more when they started leave together with Arashi and Sorata just like a cute family.
They protected each others a lot of times and Yuzuriha desire to be strong just to save Kamui make born a new Inuki, this mean that as friend and as bond she care a lot about him and Kamui too since the first thing he did was to see if she staid well.
Subaru is the respective of a big brother for Kamui and Yuzuriha is the respective of a sister, both of them have a great importance in Kamui's choice of life during the X1999 story.
Personally I think that both of them see in the respective the sibling they never have.