Hatter

Hatter does not have the reputation of a particularly honorable man in Wonderland. He's known to most as a bit of a scoundrel, peddling information to both the Monarchy and the Resistance. He's accused by Dodo of "playing both sides of the court" and attempting to "come away with two trophies". However, once he meets Alice, she seems to awaken some latent sense of nobility in him.

Background
Not much about Hatter's past is known beyond that at some time before the beginning of the film, both his father and his brother, Mad March, were employed by The Queen of Hearts. His father, having no stomach for the work the Queen had him do, poisoned himself. March, on the other hand, enjoyed his work as The Queen's assassin. At one point March went after Hatter, intending to kill him, but Hatter "got him first".

Prior to the events of the film, he's only partially a member of the resistance against the Queen of Hearts. He acts as a double agent, running information and sales for the Casino (run by the Queen) and then smuggling help, food, and other items for the Resistance-- whose side he claims he's really on.

If Hatter isn't the Hatter of the original story and "legend" (as it's implied that Wonderlanders live longer in years than humans) then it's believed that he is the descendent of Wonderland's original Mad Hatter.