I think it's fine to use truth-teller itself.
The first rule of philosophical discussion (and from the comments, we are having a philosophical discussion) is to define your terms.
Our quodlibet here is not "that which has not yet been named," but the liar to which we are proposing an opposite.
There is not a simple sense of a liar which is has a unanimous, agreed upon meaning in all circumstances. In this case, determining an antonym becomes difficult.
A liar is someone who has told a lie.
A liar is a person who has habitually told lies.
A liar is a person who usually tells lies.
A liar is a person who can't tell the truth.
A liar is a person who can't tell the truth in particular "important" contexts.
A liar is someone who has told a lie in a particular, "important" context (priestly initiation, marriage, job interview).
A liar is someone who has told a lie that is known to be a lie.
A liar is someone who has told a lie, whether the subject of the lie is a justified belief on the part of the liar or not.
A two-year-old, with developing cognitive and moral abilities, is a liar whether by "intent" or "accident".
A liar is someone who has told a lie, whether by intent or accident.
A liar is someone who has told a lie, whether by omitting the full truth or deliberately obscuring it.
A liar is anyone who has said anything "defininitive" except for the most trivial of statements, given the messy way that human brains deal with propositions, conflicting desires and loyalities, and the overloaded meanings that things in the real world have.
A liar is a robot with a broken truth-telling module.
A liar is someone who tells lies to me, repeatedly, during the period of our acquaintence.
A liar is a person who tells "little white lies" .
A liar is a person who tells "little white lies" habitually, with an occasional targeted lie to their significant other, employer.
A liar is a person who tells their lover, spouse, significant other, "I love you," without really knowing whether they mean it, on at least one day a month.
A liar is a person if their Diety has determined them to be a liar, according to The Rules.
A liar is the opposite of a whistle-blower, whether by actively covering up the truth of some circumstance, event, or condition, or by failing to communicate it to those who might be affected.
What's the opposite of a "liar"?
As a single answer, a flexible word is about the only thing that can work here. That, or multiple words for different antonymic senses.
Why not truth-teller?
I'm deriving some of this from my earlier comments and discussion around them.
My initial definition of truth-teller from the 1913 edition of Webster's was "One who tells the truth."
The two criticisms I've seen are that the term isn't contemporary, or that it's not a "single word".
I believe that it is certainly in current usage. (Certainly more so than soothsayer, even though soothsayer can also mean "truth-teller".)
Here are some usage examples from print:
Larry Summers: a Truth-Teller ...
[...] there’s something refreshing about
his persona as a truth-telling
curmudgeon who’s equally honest about
Punishing the Truth-Tellers
The Washington Post, “Bill Kellers wrongheaded attack on HuffPo”
Indeed, just yesterday, HuffPo
prominently linked to a great Times
story debunking many of the claims of
bold truth-teller Chris Christie.
Helene Cooper, The House at Sugar Beach
I would then return to Liberia a conquering hero, a famous journalist, a truth-teller extraordinaire.
Mark Clark Review of State of Play
"If there’s a hard sell in our day to day lives it’s the concept of the heroic journalist, the character State of Play director Kevin MacDonald calls his ‘truth-teller’.
There are more examples from twitter, from google books, from titles on Amazon, etc..
The accepted answer, soothsayer is as much a compound as truth-teller is. It also has the sense of 'fotune--teller' -- someone who can divine the truth, not necessarily someone who speaks truth consistently. This may be the sense intended by the OP, but isn't what I would consider the opposite of a liar.
Straight-shooter (from @Kevin) is another nice compound, which has in some cases overlapping meaning with some cases of truth-teller.
As @msh210 points out, truth-teller has had a technical usage in logic puzzles for a long time -- simply meaning "someone who always tells the truth," as opposed to the liar, who always lies (or gives not p when the truth is p).
But in common parlance, the word truth-teller usually means someone who speaks the truth in difficult circumstance, or tells "truths" which are widely unpopular (within some related constituency).