Is it permissible to believe in the existence of the Qareen (companion)? What is the extent of his influence on man?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The existence of the Qareen is established by the Quran and the Sunnah. Allaah The Almighty Says (what means): {His [devil] companion will say, "Our Lord, I did not make him transgress, but he [himself] was in extreme error."} [Quran 50:27] According to Al-Qurtubi and Al-Mahdawi ‘companion’ in this verse means the devil.

There is also evidence from the Sunnah; on the authority of ‘Aa’ishah she asked the Prophet, sallallaahu ‘alayhi wa sallam , if there was a devil accompanying her. He answered that there is indeed a devil with everyone – and even himself - but that Allaah The Almighty granted him control over his devil and he was subdued, so now he only enjoins him to do good. [Muslim] A Muslim must believe in the existence of such a companion because it entails believing in the texts of the Quran and the Sunnah.

Allaah knows best.