B’s name is not on the mortgage, the utilities etc so is losing credit proof needed should B move out. Say, A dies? Basically a roommate agreement would work. A needs a written agreement so it’s easier to evict. I’ve thought of this stuff, not as a love interest, but an actual roommate. What’s the fair rate? Roommates.com will show places rates in your area. I hope that’s net income and both are contributing to their retirement funds. Also, B needs a better job, maybe more education or certification.