A demon who offers people what they want, and collects.
Mephisto is a powerful demon from outside this dimension whose method is the pact: he manipulates people into making deals with him, and what he gives is real. Sacha Baron Cohen plays him. He is the one who gave Parker Robbins the cloak that makes him the Hood, which is the source of most of what goes wrong in Ironheart.
The series’ executive producer Zoie Nagelhout described him as a way of tying the characters’ journeys together, Riri’s in particular. Cohen worked with the trainer Alfonso Moretti to get noticeably bigger for the part in a short time, quickly enough that the costume had to be altered to keep fitting him.
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Wikipedia’s entry on him is two sentences long: a demon who manipulates humans into pacts, and one appearance. The catalogue records what the sources record and no more. What he wants with Riri is for the series to say.