Your girlfriend cannot deduct the mortgage interest (or property tax) unless her name is on the deed. To take those deductions, she must have an ownership interest in the property.
<<"she just gives me a personal check every month.">>
Is your girlfriend living in the house that you own? If so, your arrangement could be construed as renting not for profit, in which case her payments to you would be considered taxable income. If you have no lease agreement, your arrangement is called a "tenancy at will."
https://www.investopedia.com/terms/t/tenancy-at-will.asp
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
