It depends on the company you purchase renters insurance from. Some insurance companies provide a cheaper policy that does not cover your roommate. If your insurance company covers your roommate, you always need to explicitly list your roommate on your policy so that he/she can be covered.
The information in this answer is general in nature and is not legal or insurance advice for your specific situation. Policy language varies carrier by carrier; your declarations page is the final word. If you're unsure, call your agent before you need to file.