It doesn't neccessarily have to control a humanoid, though.
It could be self-animating, for one, like an intelligent dancing sword.
OTOH, it could control any kind of handy living thing capable of bearing it in some way, with its abilities determined (at least in part) by the nature of the bearer.
For example, if the artificial PC- call him Art- were a ring, it could be worn by any primate, several kinds of avians or reptiles, slipped over a cat or dog's tail, or manipulated by a cephalopod's tentacle. However, only if the form had sufficiently developed vocal chords and/or digits capable of making the proper gestures could Art cast the spells it knows.