A subtenant is someone who has the right to use and occupy rental property leased by a tenant from a landlord. A subtenant has responsibilities to both the landlord and the tenant. A tenant must often get the consent of the landlord before subletting rental property to a subtenant. The tenant still remains responsible for the payment of rent to the landlord and any damages to the property caused by the subtenant.
Generally, to evict a roommate, you must be the original tenant (or the one who has signed a lease or rental agreement with the landlord), and the roommate you want to evict must be your landlord's subtenant. A subtenant is usually someone who is renting part of your place from you and paying rent to you instead of your landlord. In this relationship, you are the "landlord" and your roommate is your "tenant."
A tenant acting in the capacity of a landlord who resides in the same rental unit with his or her subtenant may be able to evict said subtenant without just cause, as required under some local landlord-tenant laws, which vary by locality. Depending on local law, a master tenant must give written notice to the subtenant a certain number of says in advance of eviction. If the subtenant doesn't leave, it may be necessary to file an unlawful detainer lawsuit.
Under some local laws, a tenant who subleases his or her rental unit may charge no more rent upon initial occupancy of the subtenants than that rent which the tenant is currently paying to the landlord. In other words, a master tenant cannot profit off of their landlord's property.
You can't evict a roommate if you and your roommate are cotenants. You are cotenants if you and your roommate both signed the lease or rental agreement.