"Until the day I take back the manor, will you offer your life to me?"
That was a risky proposition, but from Jeremy's story, Kate saw a potentially loyal servant looking for a master. And she's exactly the master he was looking for.
We've known for a long time that Kate intended to overthrow the nefarious Shadows House, but this is the first time I've noticed her intent to also be the head of the Mirrors family. Maybe I just missed it long ago, but I previously thought that she was only interested in setting the people there free, but not actually restore her fallen noble house to power in that land.
Making Jeremy pledge fealty to her leaves no doubt about her intention to take up the mantle of the rightful heir of the Mirrors House, putting her in direct competition with Anthony and his ambitions.
The pieces are falling into place and a new ally is welcome after that devastating coup by the enemy. Kate needs better OPsec though; Jeremy could be the answer to that. She could use a spymaster who'll help her with his spying, and for once she'd not be the one who's being spied on. It's vital to keep tabs on Anthony and Edward's movements now more than ever, so they won't be some moves ahead of her this time.