You can buy it once the maid is going to outgrow her clothes and refuses to eat. Be sure you didn’t reset the maid with the lever in the basement unlocked after getting her ED with the code as this bugged everything out for me. (23571 if anyone needs it, walk all the way to the right to enter it)
I recommend using the money and experience lever only, and just buy everything in the town and go through it normally to avoid things breaking. Also do one quest at a time in the order below. For the fox, I just bought 20 of everything from the foreign vendor and it was enough to complete her full questline without having to run back and forth all the time. The knight just seems to take forever, I actually thought it was bugged. I didn’t get the dark screen bug mentioned in this thread but I was playing this game in Japanese with JP locale set, probably something to do with the autotranslated version/region setting on windows there. Here’s how to unlock everyone and also know when the quest is done, my gift to this thread after necroing with a stupid question.
Maid - Buy her, once she moves to the bed you can still give her items but her quest is done (the capacity value of items lowers which you can see in the mysterious item that has the cell phone icon)
Fox God - Listen to the bartender, then kill the guy in the bar and talk to the bartender again (character level 50 recommended) quest ends after she invites you into the bath
Knight - Be level 60 and she shows up outside your door, this one ends by talking to her (option gives a warning). Talk to her throughout the quest for some extra scenes that are possibly required to progress
Succubus/Demon Lord - Beat her in the tower at lvl 99 and she’ll come back to the empty cell, she eats the monster items which you can also buy from the guy next to the potion shop, another one where it is pretty obvious when the quest ends (you just get a loopable dialogue option)
Mage - (edit) Loveshandles got it right, thanks! I tried inspecting things but apparently it was from the wrong angle. Once you unlock the appetite enhancer you can put it in the water on her desk once a day. Again pretty obvious end, but if you aren’t sure it’s when she will stop selling you items, and there will be a portal in the corner of the room to buy from.