Violet is one of the few female clients we get in these stories. Violet is a young woman with too many suitors and a mysterious dead uncle, to boot. Her employer proposed to her, there's a guy stalking her on a bike, her employer's friend hits on her, and she's already engaged to a nice guy. She hires Holmes to figure out who her biking stalker is, but Holmes ends up uncovering a much bigger crime here.
Violet is interesting as an historical figure as well. In the late nineteenth century a lot of commentators were writing about the "Girl of the Period," a term they used for modern young women who worked for a living, lived independently, and embraced more modern behavior. These women were also often very fashionable and often rode bicycles, which were the hot new trend in the late nineteenth century. As a young 'cyclist' who works for a living, Violet is definitely a modern woman. Though she might have some independence, she still requires Holmes and Watson to rescue her in the end. If you want to read more about women in the late Victorian era, check out this article. And if you want to read more about the bicycle craze in the 1890s, and how it related to women, go here.
This guy is the nicer of the two criminals in this story. Carruthers ends up genuinely falling for Violet and proposes marriage to her for real, and not just as part of a scheme to steal her money. Carruthers is also the "solitary cyclist" of the title. He follows Violet on her bike rides in an effort to protect her from his mean business/criminal partner, Mr. Woodley. Carruthers also steps up to the plate and helps Holmes and Watson rescue Violet after she's been kidnapped. It is interesting that Carruthers hires Violet as a governess. He presumably had a wife at some point, and he has a child, though we never get any details about them.
He may be the chief villain of this story. Woodley has no moral issues with the scheme to marry Violet and steal her money. When Carruthers backs out, Woodley kidnaps Violet and forces her to marry him. The scene where Holmes and Watson burst in on the sham, forced wedding is very disturbing and violent – there's a lot of sexual undertones there, and Woodley himself comes off as a dangerous sexual predator in all his interactions with Violet. (You can read more about this in the "Marriage" theme discussion and quotes sections.) Woodley stands out for this in these stories, which mostly deal with thieves and blackmailers and murderers (often accidental ones), not potential rapists.
Williamson is a disgraced former clergyman, which is scandalous in and of itself. He gives Woodley a run for his money in the "evil" department. Williamson gets involved with Woodley and helps to kidnap Violet. He them performs a forced marriage for Woodley and Violet, though given the forced nature of it, and Williamson's own disgraced clergy status, the union won't be considered legal and binding.
This is Violet's uncle, who lived for years in South Africa and died there. He left his fortune to Mrs. Smith and Violet, which is why his two bad friends tried to scam Violet out of her money. Ralph Smith was himself likely not a very great guy if he had friends like Mr. Carruthers and Mr. Woodley, though he did seem to want to make up for his past by leaving his money to Mrs. Smith and Violet.
Cyril is an electrician and is the fiancé of Violet Smith. He does marry her at the end of the story, as Watson notes in his epilogue. Cyril has a successful career and also comes into a lot of money after he marries Violet, who becomes an heiress thanks to her uncle.
These are Violet's parents. Her father, now dead, was an orchestra conductor. The family never had much money, which is why Violet has to work for a living now. Though Violet lives at her employer's house during the week, where she works as a governess, she always takes the train home on the weekends to visit her mother.
Peter is the groom who drove Violet to the train station on her last trip there. Peter is beaten up and left on the side of the road when Woodley and Williamson kidnap Violet.