Mr. Fantastic is the superhero identity of Reed Richards. He is the leader of the Fantastic Four, husband of Susan Storm / Invisible Woman, and brother-in-law of Johnny Storm / Human Torch.
Mr. Fantastic is one of the smartest beings on Earth and his super power is elasticity; which allows his body to stretch and form into almost any shape he wants (usually for offensive or defensive situations).
His superpower was gained from exposure to the cosmic rays in space. Like all other members of the Fantastic Four his identity is publicly known to the world.