As discussed in chat:
Kenitoh is Fabelle's king, in a roundabout way. He is the one person keeping her head above water in the plot. If he decided she wasn’t worth it, or saw her do something ‘nasty’ (or a very angry Senlis went and did his regal order thing) then she’s on the run. Again. And if word gets out she was the one who assassinated Absolon all those years ago? And she’s walking on a thin tightrope with Adelais as it is. If she somehow ‘indoctrinates’ Kenitoh into thinking Fabelle is evil (not that I blame her) there she goes.
That’s just on a political level.
There’s the emotional level too.
Kenitoh, is in a way, more similar to her than anyone else; he’s a child, thrust into a role to great and dangerous for him to comprehend. Fabelle would be terrified that she not only becomes friends with him, but ends up revealing her secrets to him. She’s only ever shared secrets with her Ghost-type friends, who can certainly be trusted not to tell others or harbour anything against her for it. So for Fabelle, it’s kind of this childlike introversion; she’s scared of becoming his friend only for her to ruin it and drive him away, then leading to the political problem.