In a game I would treat them much like another demon I created a while back called the Never Was. Though I would give them a much darker edge.
These creatures are lesser Jinn also known as Personal Demons. Occult scholars suspect that everyone has at least one, maybe even several. Unlike other demons, the Qareen rarely if ever manifest in our world as physical beings. They influence their host by goading them on to do evil. The Qareen are also unique in that they work with nearly any demon. If a succubus has targeted a particular mortal in order to steal his soul, his Qareen will aid the succubus in the temptation by suggestion. The succubus gains the soul on the death of the victim. It is unknown what the qareen get. They seem motivated by evil for evil’s sake.
If the qareen is made manifest (some rare magical books detail how this can be done) they have the same stats as the person they belong too. Killing the qareen doesn’t affect the person, save for making them feel “good” and want to do good. Sooner or later another qareen will take over.
A carefully worded remove curse spell can remove a qareen, but they usually return when the character is faced with temptation again.
The Quareen uses the same stats as whatever person it is attached to. They can't normally attack and do not physically manifest. Though the days when a Qareen is most active the character must make a Wisdom-based save (or Will or Magic with Wisdom mods) to avoid giving into temptation.