At one time, Charon may have rotated at a different rate than Pluto. As Charon rotated, it was pulled first on one side and then the other by Pluto's gravity. The tidal effect caused Charon to slow down. Since Charon orbits very close to Pluto, Pluto's tidal effect is strong. However, unlike Earth, Charon and Pluto are very close in size. So, Charon was also exerting a strong tidal effect on Pluto. Finally, over a long period of time, both Charon and Pluto slowed down until they spun at the same rate. Since then, Charon has kept one face towards Pluto at all times, just like Earth's Moon. Unlike Earth, Pluto also keeps one face towards Charon.
