Initially Mrs. Clennam's servant, Flintwinch becomes a partner in Clennam & Co. He knows Mrs. Clennam's horrible secrets and uses them to gain power over her and the business.
Dickens tends to have at least one of these guys in every novel he writes. Which guys? Well, Shmoop's going to throw a couple of terms your way that are way helpful when talking about Dickens's minor characters: grotesque and monster. A grotesque is a character who generally inspires disgust in the reader (usually because of some extreme physical disfigurement), but whose underlying humanity creates some measure of empathy. We're talking guys like the hunchback of Notre Dame, the Beast from "Beauty and the Beast," or Frankenstein's monster (that's right – the doctor who made him is called Frankenstein, the monster himself has no name). Gross to look at? Sure. But so sweet once you get to know him! A monster, on the other hand, is just as disgusting on the inside as on the outside. Basically, a monster is a grotesque without a heart, like Monty Burns from The Simpsons, or Smallweed from Dickens's own Bleak House, or... well, like dear old Flintwinch here.
And what a horror this guy is. Not only does he look like his body has been screwed together incorrectly, and like he's about to be hanged by his odd neckerchief, but he is a bullying, angry old man. Flintwinch beats his half-deranged wife, Affery, tries to blackmail Mrs. Clennam, and finally steals all of Clennam & Co.'s money to vamoose to Amsterdam. But, wait, there's more! Flintwinch is also one of the novel's sinister twins, plotting his blackmail with his identical brother – another winner, who's in legal trouble for abusing a mentally disabled man under his care.