A Genie in the days before the current Demon Lord was a mischiveous spirit that roamed the lands playing tricks on the people using its powerful magic. Sometimes these spirits would attempt to trick powerful sorcerers, kings, or sometimes gods, and would be imprissoned into a container, usually a lamp but occasionally other items, and cursed them to grant three wishes for those that released them. After the wishes were granted the Genie and its lamp would disappear and reappear a far away ruin. Angry with their imprisonment they would trick those that released them and often brought about misfortune on those that rubbed their lamps.
In the age of the current Demon Lord however things are only minorly different. Genies are still mischiveous creatures that will happily trick those that rub their lamp, but often times it will end in them recieving sex. "You wish to be rich? Well then you shall be rich in pleasure." There are only three wishes they will not grant no matter how they are phrased. They will not grant a wish to kill or otherwise harm another individual in any lasting manner (pranks are acceptable, but killing or torture are a no). They will not force any already married woman or woman who is interested in another to fall in love. They will not grant any wish that will result in raising the dead, except in the case the individual has the necessary qualifications to become an undead Monster Girl.
Unfortunately Genies are incredibly rare so when found one must be very careful, for they only get three wishes before they disappear from the person forever. The only know solution is to wish for the Genie's freedom, however in doing so the Genie will often times rape the man that released her relentlessly in her gratitude. Often times they will then take the one that released them as their husband.
Should one find themselves married to a Genie one can expect a life of luxery as the Genie, now free to grant wishes as she pleases, will often times bathe their husband in the lap of luxery. Their lamp becomes something of a portable home, the interior of which is magically created to best suit both husband and wife.
Genies have been known to remoddle their looks in an instant to best please their summoner. They will sense an individual's preference the moment their hand touches their container and so when released they will have the most pleasing appearance possible.
Interestingly it has veen noted that while women are capable of find the lamps of Genie's no matter how hard they rub the Genie seems to refuse to come out.
I think it could use a little work playing into the monster's ethos, y'know? Simply put, like any other spirit of mischief, genies can be jerks, but they usually play into or play upon certain type of element, greed. In my opinion, while a genie can be sweet, they should always seek to be the posessor and not the posession. Therefor it would make sense for a genie to not just have one husband, but a whole harem of them, using them to cater to her own wishes and desires within the realm of the lamp. However that's not to say that those who join her harem would be unhappy, with the lamp she controls all, and would happily grant any wish for any of her husbands, and multiply herself so that none would ever be lonely. Simply put, she loves having sex and showing off, and the more in her harem, the more she can do of both. And when summoned from her lamp, if she takes a liking to her summoner, she will do everything in her power to try and seduce the one making the wishes and should they wish to have sex, she would naturally insist on doing it in her "bedroom" where she would have more power to further seduce and convince the new addition to her harem. But that's just what would be my take on a genie monster girl.
As a side note I'd have a varient of much more docile and monogomous ring genies, though they'd still show a sign of marriage, the ring natually being magically affixed to a place that *ahem* any monster girl would find.