Mister X

Mister X is a Letter Boy from The Letter People. He is, at first, an outcast because he (unintentionally) makes things go wrong (including getting things wrong itself). He puts Xs on something (specifically, as shown, this makes Mister B's voice hoarse, makes Mister H's hair look like short, curly hair, and makes Miss U's nanny goat make various sounds (including vehicle types and speaking) instead of maaing). Nardo takes care of these problems by removing the Xs from Mister B, Mister H, and the nanny goat.

When Nardo meets Mister X, he confronts him, but forgives him, and notices that Mister X was where the letter had been shown (Nardo's teacher marked any wrong answers with an X when he was a child). Mister X warns Nardo against touching his Xs because it will make things wrong. It is shown that because his Xs make things go wrong, he never had any friends. He warns Nardo to go away, but Nardo tells him he is on his side.

Nardo introduces Mister X to Mister K and Mister S. Miss A had told them a solution to help: to stand together until they make a squoosh sound for Mister X alone. Nardo tells Mister X that he will always end a word so as not to make words look wrong, and then proves Mister X is wanted in Letter People Land by showing him words that end with his letter: "Six," "box," and (this is the hard one), "fox." With help from Nardo, he calls in Mister F to go into the Starting Clue Box, Miss O to go into the Catching Clue Box, and Mister K and Mister S to get into the Ending Squoosh Box, but the latter two refuse and tell him to use the Ending Clue Box himself and hold the badge of cooperation, so that Miss O will use her "Obstinate" starting sound.

Mister X then asks Nardo to touch his letter to see if things go wrong. Nothing does. So, as a favor, Mister X decides to use his letter as a warning to stay away from anything dangerous, like bottles of poison and railroad tracks.

Mister X is similar in appearance to the late Richard Pryor.

Mister X is one of those Letter People who do not appear in any more than one episode or sing.