The key kinda "guides" Catt (and Flickie) through her spiritual/personal growth; enabling her to "find herself". It responds with her certainty of herself, her confidence, her ability to perform magically, etc. It will actually close off her magic potential if she's too unstable earlier on, making unexpected stress a great exploitation. Like most mages, being directly injured or distracted cuts off the spell, resulting in nothing happening, and she has a very bad attention span. Flickie is able to work with theses hurdles and helps her bring out her best potential in combat (unlike others who've tried) so the key really "likes" him.

The key eventually selects him as it's master (depicted elsewhere with some Ozzy lyrics) which is Catt transferring a lot of trust into him. The key is her greatest weakness but has a lot of advantages and vulnerabilities.