Short answers, pulled from the story.
Hydra was discovered using the Hubble Space Telescope. The discovery images were taken on the 15th and the 18th of May 2005, with the find confirmed by archival Hubble images from 2002. The discovery was publicly announced on the 31st of October 2005.
Hydra measures 50.9 km along its longest axis, 36.1 km along its middle axis, and 30.9 km along its shortest axis. It is the second-largest moon of Pluto, slightly larger than Nix.
Hydra was named after the Lernaean Hydra, the nine-headed serpent of Greek and Roman mythology. The nine heads subtly reference Pluto's former status as the solar system's ninth planet. The International Astronomical Union approved the name on the 21st of June 2006.
During its flyby on the 14th of July 2015, New Horizons imaged Hydra from a closest distance of 231,000 km at a resolution of 1.2 km per pixel. The images revealed brightness variations, a dark circular feature 10 km across, and confirmed the moon's highly reflective water-ice surface with a geometric albedo of 83 percent.
Hydra has a geometric albedo of 83 percent, making it highly reflective. This is caused by relatively pure water ice on its surface. One explanation holds that micrometeorite impacts continuously eject darker material, keeping the surface fresh and bright.
Hydra is not tidally locked to Pluto, and its rotation is destabilized by the varying gravitational pull of Pluto and Charon as they orbit their common barycenter. Hydra's irregular shape adds further torques, causing its rotational axis to flip regularly. At the time of the New Horizons flyby, its rotation period was approximately 10 hours and its axis was tilted about 110 degrees to its orbit.