how to get billy a girlfriend?
If you max out Danni's fetish by repeatedly asking her personal questions, she becomes a very loose version of a girlfriend. You go on dates I think on Tuesdays by going to the café and get to fuck other girls with her consent (or at least I saw one scene with multiple girls). I haven't finished the full line of events and have only triggered it once though. For reference I have the radioactive mod installed, though I'm not certain it's required.
You may also need to max out Steph's fetish because there is a whole interaction with the two before she agrees which is triggered by doing one of those extra dialogue options (the ones that give love), though I pressed literally all of the buttons on both characters so I don't remember which one and I have no idea if she's actually required to be interacted with at all.