Karissa is the night manager at O'Jacks whom Ellie worked for and became friends with at her second job.
KK is revealed to be Quinn's first roommate in the comic "The First Roommate".
The author mentioned here, that KK will be main character in final arc (and in the final cast panel) and Ellie will just cameo for that arc. So we'll be finding out more about KK later.
First appearance here.