The central character is otherwise known as the main character or the protagonist. Here, despite Miss Hinch being the title character of "Miss Hinch," the central character is most definitely Jessie Dark.
Why is the protagonist Jessie Dark instead of Miss Hinch? Because Jessie Dark is the reporter, bent on fighting crime, that we follow in this story. Miss Hinch is simply the criminal being followed at this time. Through the story, Jessie Dark and Miss Hinch are both in disguise. The entire story is about a "clergyman" and an "old woman" as they traverse the subway in New York City, enter a restaurant, pass cryptic notes to waiters calling for the police, and continue following each other.
“Jessie Dark,” it appeared, was one of those most extraordinary of the products of yellow journalism, a woman “crime expert,” now in action.
Just when the reader thinks that the "real" Miss Hinch (who we now think is the old woman) has fallen to her death while Jesse Dark (who we now think is the clergyman) offers condolences, the "old woman" reaches up and uses her hat pin to rip the face of the clergyman (including the beard) revealing that it is, in fact, the clergyman (and not the old woman) who is Miss Hinch and the old woman (and not the clergyman) is actually Jesse Dark.
In conclusion, the reader probably notices that there is an irony in the idea that the title character is not the main character. In fact, Miss Hinch is the antagonist of the story: the criminal always trying to get away.