The queen of Attilan, and the only person who can hear what the king is saying, because she is the one who speaks for him.
Medusa is Black Bolt’s wife and the queen of Attilan, able to control and move her hair. Serinda Swan plays her.
Her real function in the story is translation: Black Bolt cannot speak, so she is his voice, and the marriage is written as a working partnership rather than a decoration. The series then removes her power in the opening hour for production reasons that were widely reported, which is the single decision that did it the most damage.