In the comics, Mephisto is obsessed with enslaving souls for his own nefarious purposes, so is it possible that he is the demon she made a pact with? Perhaps, although, Madisynn describes a demonic fella hilariously named "Jake," which doesn't really line up with any pseudonym Mephisto traditionally operates under. This is by no means a clear-cut case, as attorney Jen Walters herself would be the first to admit, but anything's possible.