A Qareen, according to Islamic literature, is a jinn. Qareens are unique to each individual and literally means 'constant companion'. A qareen pushes a person to do evil things and to disobey Allaah, with the exception of Muhammad. Someone to whisper to that person "in the light of day and the darkness of night".
Allaah says (interpretation of the meaning):
"His companion (qareen) will say: ‘Our Lord! I did not push him to transgression, (in disbelief, oppression, and evil deeds), but he was himself in error far astray." This is the belief that the Qareen does not force anyone to do evil things, but to seduce those who are susceptible or want to be seduced into doing those dark things.