Charon

largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km.

Charon and Pluto orbit each other every 6.387 days.

Charon is tidally locked to Pluto but unlike the our Earth - Moon system Pluto is also tidally locked to its moon Charon, meaning that Charon is only visible from one half of Pluto and can not be seen from the 'Far' side of Pluto at all, likewise for Pluto from Charon.

Charon is also just over half the size of Pluto.

Charon, Pluto's Companion: What We Learned from New Horizons

Dr. Ross Beyer (SETI Institute)

Pluto’s large moon Charon turned out to be far more interesting than astronomers expected. Pluto was the star of the New Horizons show, but the features on Charon’s surface tell a fascinating tale of how icy worlds could form far from the gravitational influences of the giant planets. There is evidence of a world-wide sub-surface ocean early on, and of global expansion as that ocean froze solid. Dr. Beyer is your expert (and humorous) guide through this story of formation and change in the frozen reaches of the outer Solar System.