Legal Question in Landlord & Tenant Law in California

The question is how did the person come to be a roommate and is the person paying rent? Another question is are you the property owner or one of the lessees who no longer wants this particular other person to have access to the property? If the person is paying rent and has been in possession of the leasehold or at least their share for more than 30 days, then you have acquired a co-tenant who is fully entitled to possession and who may only be removed by appropriate legal recourse. In other words all of the tenants may have to terminate their agreement and move (multiple tenants are basically a partnership). If you are an actual owner of the property, then you may want to serve this person a 30-day notice.

I recommend looking at some options including mediating a move out agreement. Good luck with this.