there are many subtleties about Grim that make me think he once lived with humans like a pet, or at least he wasn't living in the wild like a feral cat for very long before his infiltration of Night Raven College.
where did he get his ribbon? how has he gotten by when it rained in the past given how he reacted to it in the prologue? where did he learn to read and write? how did he learn what a school even is?
and many more small details of his behaviour that I can't quite put into words