Siblings which no force in the world could ever separate. That's what would describe them best I think. In my AU, Kate is one of longest active proxies, being under the influence of the Faceless for over 10 years now, so she has her duties to show the newbies (when there even are some) the ropes. Toby was one of those newbies and a pretty young one at that, 17 years old wasn't much especially when it came to any survival experience, ESPECIALLY for someone like Toby. So when Kate, a 25 year old at that time, got the info that a damn teen would live with her for some time to get used to his new life, she definitely didn't expect to get along so well with him.
They don't have all too much in common, never had but there was this weird force that somehow made them grow into a pair siblings rather than co-workers or friends. They bickered over the most mundane things, they had petty fights, they comforted each other as much as two serial killers could and had the best time together. Toby drunk his first liquor with Kate, killed his first victim, after his dad, with Kate and of course Kate was the first to notice when Toby started crushing on Nat. Even though they stopped living together a good time ago, they would regularly eat dinner together, show up uninvited at each other's cabins etc...To be honest, Kate also reminded Toby a bit too much of Lyra, yet was still so different to her for him to only see her as a replacement for Lyra. She was an additional older sister of which he woul've never guessed would come into his life but oh well.
(not proof reading ts btw idgaf if there are any grammar mistakes)