I am actually in the process of buying a duplex in order to do some house hacking. One side is currently rented at $900/month. I am moving into the vacant one with my girlfriend of almost 2 years. She also will not be on the mortgage, deed, or putting any money towards the down payment. We have agreed that once I figure out what all my expenses are after the rent comes in, we will split 50/50. However, since she doesn't own it, I would be the one paying to fix the major systems if they break. Same as if I was the landlord. Any normal repairs that would get charged to the tenant, we would just split 50/50 also. In the end, she pays less than if I actually charged her rent and it also splits my bills in half. So it is a win win for both of us. We can talk about these kinds of things with no problems or arguments so I am completely comfortable doing this with her. If you're not 100% sure that this girl is sane, I would not have her move in. And after 6 months, I don{t see how you could be 100% sure. If we end up getting married, I'll just add her to the deed. At that point I won't care that she didn't contribute to the down payment.