Can I claim my live in girlfriend as a dependent and file as "head of household" if I pay all household expenses?

[DoninGA]

You cannot have Head of Household filing status if the dependent claimed on your tax return is not related to you.

You may be able to claim her as a non-relative dependent under the Qualifying Relative rules if she meets all the requirements under the rules.

To be a Qualifying Relative -

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.

2. The person either (a) must be related to you or (b) must live with you all year as a member of your household. 

3. The person's gross income for the year must be less than $4,150 (social security does not count) in 2018

4. You must provide more than half of the person's total support for the year.

5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.

6. The person must not file a joint return with their spouse.