Mackenzie "Kenzie" Kostova was one of Cetri's friends when they were younger. That had a significant age gap in human years, Cetri was 7 and Kenzie was around 13. She was essentially Cetri's older sister figure.
Kenzie is very standoffish, but was willing to be friends with Cetri because their parents knew each other. Before Cetri moved and eventually became attached to their cousin Lilith, Kenzie was the one they were stuck to, always at her hip, copying her, trying to be just like her because in their eyes, she was the best and just like Cetri. A hybrid. Part witch and part vampire wasn't too uncommon, but she was the only other mixed child Cetri knew and they admired her for it, they thought she must be right about everything! Unfortunately she wasn't very happy about her lineage and she especially disliked how positive Cetri was about them both being so,, unique.