Qareens (Arabic: قرين) qarīn), according to Islamic literature, are spirits permanently assigned to each person during his/her life. The companion can be either good or evil.
Etymology
Qareen literally means 'constant companion'.
Behavior
The qareen, being a jinn-type creature, if not guided by his human companion on the right path, becomes a shaytan by the command of God.
Story
The Messenger of Allah (sallallahu alaihe wa-sallam) once told his companions:
“Every one of you has been assigned a companion from the Jinn.” They asked:
“Even you, O Messenger of Allah (pbuh)!” He replied: “Even me, except that Allah has helped me against him and he has submitted. Now he only tells me to do good.” [Saheeh Muslim] (The Shaythaan of Prophet Muhammad was converted to Islam.)