One word noun for "truth-teller"?

Solution 1:

In a word, the answer is "no". In the same way there is no direct antonym for murderer, thief, car-jacker, etc.

Saying that, there did use to be a word: Soothsayer. (In the mid-14th century it meant, "one who speaks truth".)

Solution 2:

Straight-shooter comes to mind.

Solution 3:

I cannot think of a simple antonym for liar. This thread postulates that this is because there is no absolute condition for truth.

Idealistically, one assumes that another is always telling the truth, and if he is not telling the truth he becomes a liar. In essence, we may prescribe that telling the truth is a "normal" behavior and thus a word has not been given to describe it. Similarly, there are no simple antonyms for killer or robber.

Solution 4:

In logic puzzles about people who always lie and people who always tell the truth (of which there are many), the latter are usually called truth-tellers. However, sometimes one finds in that context truthers, a word that has since come to mean something else also.

Solution 5:

Candid? As in: "He's a perfectly candid fellow; he told the Emperor exactly what he thought of his new wardrobe." Frank would also work there.