WMG / Varjak Paw

In the end, Holly is possessed by the spirit of now-deseased Sally Bones
. That's why one of her eyes is blue.

Jalal and Sally Bones aka Saliya of the North had a history.
They were lovers, and he taught her the Way. Then she found out about his bidding relatonship with the other cat, who was the Paw's female ancestor, got jealous, and killed him at the first opportunity (already after he sired a child with the said cat). Poor Varjak, he spoke to someone who banged his ancient ancestor.