Dong Quixote's stinking crust-sock flail, weighted with nickel, which apparently speaks in his mind.
Spunky Jefferson is Dong Quixote's crust sock flail, which he keeps dangling over his shoulder with the nickel end of the stinking sock near his ear. Carl is pretty sure the thing is talking to Dong in his mind, and Dong refers to it as 'Mr. Jefferson,' quoting its opinions on what happens to people in high-stress environments.
When Dong begins relaying the sock's theories about Princess Donut, she snaps that he will not be discussing her with his disgusting sock because it makes her very uncomfortable — to which Dong replies that Mr. Jefferson said she would say that.
A rigid, foul-smelling sock stiffened into a club, its far end weighted with a lump of nickel.
Dong Quixote's weapon and unnervingly perceptive advisor.
Also appears in books you've already read — spoiler-safe to their endings: