Maximus

Maximus is Black Bolt's younger brother, an Inhuman genius whose Terrigen gift is mental — telepathy, mind control and a capacity for invention that rivals Attilan's best engineers. He is also the reason his parents are dead: his collaboration with the Kree caused the accident that killed them, and Black Bolt's response was the destruction of the Kree ship, which killed them too. Maximus has never processed it, and the resulting instability is genuine rather than theatrical.

What makes him work is that he is not merely jealous. Maximus believes he would be a better king, and he is often right about specific policy — he has argued for Inhuman engagement with Earth, for abandoning genetic caste rules, and against Black Bolt's silences. He has seized the throne of Attilan repeatedly, built weapons capable of levelling cities, and each time been exiled rather than executed, because his brother cannot bring himself to end it. Iwan Rheon's version in Marvel's Inhumans made the class-war argument explicit, which is the most interesting thing that show did.

Maximus debuted in Fantastic Four #47 (1966) as Attilan's usurping regent and has been the Royal Family's internal wound ever since. Paul Jenkins and Jae Lee's Inhumans (1998) gave him his best material — a genuinely persuasive case against his brother's rule — and later runs from Silent War through Inhumanity kept him cycling between coup, exile and reluctant alliance.