Essentially, he is a demon who loves to bargain for the souls of heroes.
"Marvel is talking about setting Mephisto as the prime antagonist for the Defenders. Some may say that this villain wouldn't fit the TV shows but that's why they're incorporating Iron Fist – to set up the supernatural element. This would also help set up the next phase of the Netflix shows, as Moon Knight, Blade, and Ghost Rider are being considered."
The Hand, more than anything, seems to be a likely choice as the antagonist for The Defenders. Daredevil really set this up back in 2015, and it does look like some sort of supernatural threat will be making its way into Hell's Kitchen, potentially starting this "war" we heard Stick tell Matt about in the series. There may also be elements in Luke Cage and Iron Fist that will add to that villainous threat.
These Marvel/Netflix shows have really been going out of their way to stay grounded in reality and have a gritty, "realistic" tone to them. Granted, Nobu's immortality and the Hand victims essentially turning into zombies kind of went beyond the established, grounded tone.
Mephisto makes more sense within the Marvel Cinematic Universe, going up against a hero like Doctor Strange. That possibility is rumored to happen at the end of this first Doctor Strange film coming up in November, or in a sequel. It's a bit of a stretch to think of Mephisto walking around Hell's Kitchen, dealing with Charlie Cox's Matt Murdock, given the tone of the Netflix universe.
However, if these rumors about Ghost Rider being brought into the Netflix world are true, Mephisto being brought in would actually make sense. The two kind of go hand-in-hand, if you are considering bringing just one of those characters to Netflix, introducing the other one isn't any more absurd. And, if that's the case, then this Netflix section of the MCU is going to get a lot bigger and a lot more complex. Blade will bring along vampires and that whole mythology, then you've got demons and spirits with Ghost Rider, and Moon Knight is a mentally ill billionaire who becomes the avatar of the Egyptian moon god Khonshu on Earth.
That kind of world actually sounds like a perfect fit for someone like Mephisto. But, again, these are all just rumors and we'll have to wait and see.