Jeffrey's Demon was the Demon who had possessed Jeffrey many years ago, possibly around 2007. Jeffrey had been forcibly possessed and may have been possessed by one of the original demons who broke out of Hell in Wyoming. Jeffrey's Demon was a powerful demon, he could possess people, had immunity to pain, was a masochist, sadist, mind-breaker and telepath, as well as having all basic demon powers. He also had a great hatred of women, as he needed them to be sacrificed to be freed.
Jeffrey's Demon was interrogated by the Winchesters because he knew where Lilith was, and the Demon could give them useful information, but the Demon escaped their torture. Jeffrey's Demon got back into Hell and lay there for many long years, all through the Apocalypse, in fact.
With the Apocalypse over and Lucifer defeated, Jeffrey actually missed his demon possessor and desired possession again. He began killing women to free the Demon. Jeffrey was successful in the summoning, and welcomed his Demon friend. The Demon however took the liberty of possessing another man rather than Jeffrey and angered him. The Demon let Jeffrey drop his guard and let him be killed by Dean; but before being banished back to Hell the Demon told Dean how Jeffrey would return stronger.