Qareen in Arabic means accompanying person. Qareen can be a Shaitan as it is mentioned in Quran ( He who neglects remembering Allah, We will keep with him a Shaitan who will be accompanying him (Sura 43, verse36).
Qareen can be a bad companion who makes bad things in the shape of good. (Sura 41, verse 25).
There is no evidence that everyone is born with a Qareen, though it is mentioned in Some Sunni books to justify their claim that human being is not free in his will.
'The followers of Ahlul Bayt (AS) believe that every human being has full free will to do or not to do that is why he is responsible about his acts.
Those who forget about Allah (SWT) and neglect remembering Him, Shaitan will be with them as a Qareen.