If you are legally unmarried your choices are single or head of household. But, to claim HOH status, you must pay more than half the household expenses AND have a qualifying dependent that lives in your home more than half the year that you care for. This is usually your child under age 19, or under age 24 while being a full time student. An adult who is not your parent (or in-law) and not your child (or niece or nephew) is never a qualifying person for HOH status even if they could be claimed as a tax dependent.
So unless there are other dependents at home, you file as single.