A law clerk at the office of Kenge and Carboy, Guppy is Esther's first suitor. He is also the first to realize that there is some connection between Esther and Lady Dedlock, and the first to investigate the novel's mystery.
With a name like Guppy, this is probably not going to be too serious a guy. Guppy provides pretty reliable comic relief for the novel. From his crazy, out-of-left-field proposal to Esther, to his following her around with puppy-dog eyes, to the way he says everything as if giving a deposition, to him freaking out that she might still want to marry him after her face is disfigured, most things he does are meant to just be straight-up funny.
Still, Guppy is also one of the novel's unofficial detectives. He stumbles on the fact that Esther and Lady Dedlock really look alike, and he's pretty good at putting together evidence and clues. What separates him from the other investigators is motivation. He's not a pro, and he's not doing it for money or power – instead, he's working on Esther's mystery just out of his feelings toward her. (Which are what, exactly, by the way? Love? Lust? We're meant to draw our own conclusions.) This is why when she asks him to stop trying to figure everything out and to give up his detective work, he immediately does. Unlike many of the other detectives in the novel, he is not actually crazily obsessed.