Until 1978 it was believed that Pluto was much larger. Through a telescope on Earth, Charon and Pluto look like they are the same object. However, in 1978, Jim Christy discovered that what we used to think was one object was really two. Astronomers probably thought they were one object because they are so close to each other. Charon is also almost as big as Pluto.
Charon and Pluto keep the same face towards each other at all times. This means that imaginary aliens living on one side of Charon would never see Pluto, and imaginary aliens living on the opposite side of Pluto would never see Charon.
Charon is named for the god who ferried the dead across the River Acheron into the underworld.
Pluto's second moon is called Nix. It is much, much smaller than Charon.