That's pretty damn creative. Kageshini isn't bound to a single mirror, though; she just uses reflective surfaces to interact with our world. That was showcased when she appeared in a window at the school.
Still, love the idea!
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Hi I had another idea that I’m not going to use because I could not create the image I wanted for a female golem. Have one of your students paint the bolder’s (aka Breast) of the golem while she sleeps during the day then wakes up at night to eat dead wood in the forest. The golem is the reason you never find any dead trees laying around on the ground. I’m pretty sure you could make a fun story even if it’s just a side character. Well take care