Mister Fantastic

Reed Richards is the smartest man in any room he enters, and the Fantastic Four films have consistently framed that as a problem rather than an asset — he solves for the universe and forgets the four people standing next to him.

His arrival in Doomsday brings the single most important relationship in the film with it. Reed and Victor von Doom are the definitive rivalry in this material, and it is the reason Marvel could cast Doom as a genuine lead rather than a closing-act obstacle.

He crosses over from his own universe, which the film uses as its mechanism for putting three separate continuities in one story.