You do not use that form at all to claim a girlfriend. That form is used by divorced or never married parents who share custody of a child.
If your GF lived with you the WHOLE year and had less than $4400 of income (Not counting Social Security) then you might be able to claim her as a qualified relative and you will get the $500 credit for other dependents, which will not be a refund, but will reduce tax you owe.
CREDIT FOR OTHER DEPENDENTS
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
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