The villain of this story. He tried to kill Holmes in a mountainside dodgeball tournament, substituting rocks for playground balls, and then he came back to London and assassinated Ronald Adair with an air-rifle contraption. Moran was the one who was cheating at gambling, which Adair apparently found shocking. Then Moran tried to kill Holmes, but was caught instead. Interestingly, he's one of the very few criminals who doesn't confess everything to Holmes at the end. Moran also stands out for being a formerly respectable guy turned professional criminal. He inspires a discussion between Holmes and Watson about what makes someone turn to a life of crime.
This is the young aristocrat/gambling fan who is murdered. His is a classic "locked room" mystery, since no one can figure out how he was shot. Until Holmes arrives back from the dead, of course. Turns out, poor Ronald was killed because he found out another man was cheating at gambling and planed to be a tattle-tale.
He is already dead by the time this story begins, but he plays a major role in the ensuing events. Moriarty was Holmes's arch-nemesis until Holmes managed to kill him in a story published nearly a decade previously, "The Adventure of the Final Problem." (You can read more about that in the "In a Nutshell" section). Moriarty was a criminal mastermind with a dangerous gang that's still after Holmes.
This is Sherlock Holmes's brother. Though he never appears in the story, Holmes mentions that his brother was the only person who knew he was still alive. Mycroft took care of Sherlock's affairs while he was gone and protected his secrets until Sherlock decided to rise from the dead.
Sherlock Holmes's rather unfortunate housekeeper. We say unfortunate because Holmes nearly gave her a heart attack when he popped back up from the dead. During his "death" she cared for his house, as per Mycroft's instruction. Mrs. Hudson appears off-screen in some other stories. We're assuming that she's the one who answers the door when police officers/people on the run from the law/suspects/stressed-out clients come calling. The woman needs a raise.
In a newspaper article reporting on Adair's murder, we learn that he is the son of the Earl of Maynooth and that he lived with his mother and his sister, Hilda. We never meet any of these people, but details like this help Ronald's murder have more of an emotional impact.
The fiancée of the murdered Ronald Adair. Again, details like this help us to get more invested in Adair's murder.
These are men that Ronald Adair gambled with while he was alive. Stylistically, the quick mention of these men is interesting. Watson mentions these men as if his audience would actually know who they were, which helps add to the realism of the Sherlock Holmes tales. These stories are written as if they all really happened and involved real people.