Che `the Metallicar` V. Impala is from an AU splitoff of current season (S3 that is) near the end (that hasn`t happened yet).

Sam had been trying to figure out how to get Dean out of the whole soul-selling thing, but couldn`t tell him outright, because of the whole issue with "oh yeah, I`ll drop dead again." He finally figured out that if Dean had a different demon protecting him, whoever it was who had the contract, crossroads demon or their boss, wouldn`t be able to lay claim to him.

He was smart enough not to go and get himself possessed again, but figured out how to summon one of the lesser imps right smack into the body of the Impala, of course, that`s where things didn`t exactly go as planned, because, see, even without sentience, the car had personality, and it was a more dominant personality than whatever attitude the demon had (that many years around Hunters will do that to a girl!) and it came through and came out on top.

Dean didn`t know about it, probably won`t learn about it until the deal comes due. And more importantly, Che herself doesn`t know about it, partly because it's difficult to say exactly how much of the demon-part made the transfer from car to human during the transportation storm, if any actually did at all. At the very least there would be some residual energy that those in the know would be able to spot.

UPDATE!  POST TIMEY WIMEY PLOT!

All the above information IS still valid, it's just that Che herself doesn't know any of it, because during this plot she was bounced backwards along her own timeline to 1977, back when she belonged mostly to John and a little bit to Mary (who as far as she's concerned is the only woman good enough for John)

She does remember bits and pieces, the same as newcomers sometimes remember things from the people they replaced.

Info

Played by Sterling journal is Lady_Metallicar and her PB is Melanie 'formerly Scary Spice' Brown.