I've done some research, and I have a 'Lodger' rather than a 'Tenant'. Apparently a lodger is someone that lives in the same building as you and you charge them a monthly fee (aka rent). A tenant is someone that lives in a property or apartment/townhome unit not occupied by the owner.
Apparently there are different set of legal rules that apply to straight up tenants and lodgers. Tenants tend to have more rights than lodgers.
I'm trying to just deal with it, but if her visits continue at this present frequency, I'm gonna have to address this with my roommate with the intention of asking him to leave if he doesn't give in at all.