Who is Sir Henry, and why is he important to Mortimer?

Sir Henry is Sir Charles Baskerville's nephew. He is also the heir of the Baskerville estate. He is important to Mortimer because Sir Charles was one of his closest friends. In addition to their friendship, Mortimer has a manuscript that details a history and legend concerning the Baskerville family and the "hound of hell." Dr. Mortier also mentions Sir Charles' concerns about the legend of the hound, as well as his fear. Lastly, when Dr. Mortimer investigated the place of Sir Charles' death, there were giant, hound footprints. Thus, Mortimer turned to Holmes for assistance.