They do not have to be a relative but you must be financially supporting the roommate. You can use the IRS Interactive Tax Assistant to decide. IRS says
You may be able to file as head of household if you meet all the following requirements.
You are unmarried or considered unmarried on the last day of the year.
You paid more than half the cost of keeping up a home for the year.
A qualifying person lived with you in the home for more than half the year (except for temporary absences, such as school).
