Charon was discovered by Jim Christy in 1978. Before the discovery of charon it was believed that Pluto was much larger, because the images of Pluto and Charon were very blurry and seemed to form just one planet.

Charon is very unusual because it's the largest moon in respect to the planet to which it belongs. Pluto and Charon are more a double planet then planet and moon.

Charon's radius is not well known as well as it's  density and mass.
Charon seems to have a very low density, but it's precise composition is unknown. 
The satellite is probably covered with a thin, icy layer. This is very different from Pluto's crust

Charon is unlikely to have an atmosphere.