The Truth

Truth is a metaphysical being who exists everywhere and eternally. Truth oversees everything in the universe, and regulates all Alchemic exchanges in the world. Truth explicitly reveals itself when alchemists attempt to "Play God" by attempting Human Transmutation. Seeing Human Transmutation as an unfair exchange according to the equivalent exchange laws of Alchemy, Truth will bring the alchemist into the Gate and forceably take their most valued attribute, or something of value that represents their anticipated relationship to the person they are trying to create, or resurrect. Truth will accept otherwise impossible exchanges when a Philosopher's Stone is used, as it or part of it will be used for the exchange instead.

Depiction
Truth is depicted as a white, featureless version of whomever passes through the Gate; this is because truth serves as one's own conscience. Truth can be seen being in possession of items that it took from the Alchemist when they opened the Gate. In Edward's case, Truth appears as a featureless young boy, but later possesses Edward's arm and leg which were paid as an equivalent exchange for seeing Truth, and getting Alphonse's soul back. In Alphonse's case, Truth possesses his entire body, and appears as such.

Due to their relative appearance, when Edward and Alphonse Elric each appeared before the Gate, Truth vaguely resembled a young boy and when Izumi Curtis appeared, Truth's shape reflected that of a young woman. It appeared as a vigorous man to Roy Mustang, and when the Dwarf in the Flask entered the Gate, he appeared as the Dwarf's original form when he lived in the flask.

Manga and 2009 Anime
Truth introduces themselves as the being some people call God, the World or the Universe, at first seeming welcoming and jovial to its guests before calling attention to their hubris and opening the Gate, which has forced them inside earlier and now forces them outside.

According to Father, Truth's sense of toll payment is often based on dramatic irony, oddly unfitting of Equivalent Exchange, which Truth upholds. Due to Edward Elric opening the gate in an attempt to bring his mother back through human transmutation, Truth takes Edward's left leg and his brother Alphonse. In this case, the irony was that Edward had to pay the leg he stood on and the only family he recognized as having left. Edward paid his right arm to get Alphonse's soul back, which Truth recognized as an equivalent exchange (as one could consider Alphonse being Ed's 'right-hand man' if you will; furthermore, Truth let an exchange on human souls occur, something Truth would usually never do, presumably out of pity for Edward). As a result of Alphonse's own hubris and cooperation with Edward, however, Alphonse (fittingly, in Truth's view) lost his whole body, and the ability to feel his mother's warmth. Izumi Curtis attempted to bring back her stillborn child through human transmutation, and Truth took some of her internal organs, presumably including part of her reproductive system, leaving her incapable of ever becoming pregnant again. An unusual case is Roy Mustang, who was forced to open the gate by Pride (with the aid of Wrath). Regardless of this conscious decision to not perform human transmutation, Truth still punished him by taking his eyesight (although since he did not open the gate intentionally, he retains his eyes.) The irony in this case was that Mustang would not be able to "see" the future he would make for Amestris if he were to become Führer. It is imperative to note, however, that Truth also took away most of Pride's strength, since Pride was the one who forced Mustang to perform a human transmutation, thus rendering Pride easy to defeat. In nearly all these cases, the damage done by Truth cannot be repaired even through Alchemical means; even Van Hohenheim, a living Philosopher's stone and one of the two most powerful Alchemists in the series, stated that he could not regenerate Izumi Curtis's missing organs. Mustang's case was unique however, as he later regained his sight with the use of a Philosopher's Stone. It is likely that this was due to the fact that Mustang was forced to open the Gate rather than making the choice to do so as Edward, Alphonse and Izumi did.

If one chooses to disbelieve the vain and egotistical Father, however, then Truth takes their payment based on Equivalent Exchange, as in the case of Izumi's organs. Edward and Alphonse were somewhat unusual in that their array specified that a soul be bound to their creation, and thus Alphonse was taken as material. Accepting his body as payment, Truth granted a larger share of the contents of the Gate to Edward, the alchemist leading the transmutation, respecting the price he paid, and Alphonse left the Gate remembering nothing of the knowledge, which he had not paid for. Roy Mustang, forced to open the gate by Pride, remains an unusual case, but it may be assumed that as the one who performed the transmutation was Pride, the majority of the Toll came from Pride, leaving Roy (save for his eyesight) unharmed and able to fight, while Pride was greatly weakened to the point of extreme fragility. If Truth punishes people based on Equivalent Exchange, then Roy lost his eyesight as a price for "seeing" the Truth.

Though Truth appears in Edward's dreams, mocking him, Truth's next true appearance is in Chapter 53, when Edward crosses to the true Gate from inside Gluttony's internal imitation. Truth appears amused that Edward has not come to retrieve anything that he lost on that day four years ago. Interestingly, Truth does not seem to appear before Edward as he discovers Alphonse's Gate and the body that sits before it, but - as the body speaks to him even while its soul is in the mortal plane - it is likely that what Edward sees and speaks to is Truth speaking through Alphonse's body, especially since the body is seated in the same manner as Truth whenever it appears.

In Chapter 102, Father discusses the irony behind the works of the 'Truth', who took Ed's way to 'stand by himself' and his 'only family', Al's body so he 'cannot feel the mother's warmth as he craved', Izumi's 'capacity to nurture the seed of life' and, now, with Mustang 'depriving the man who had a grand vision to save his country of his eyesight, denying him to see what his beloved nation will become'. In Father's perspective, the Truth is cruel and sadistic, whose only ambition is to make people suffer.

In Chapter 107, Truth appears once more before Alphonse once his body and soul are finally reunited. Truth returns Edward's arm in exchange for Alphonse's soul, and questions the younger Elric if his brother will return for him and what he'll sacrifice.

In Chapter 108, Truth appears before Father and mocks him for trying to claim the power of God for himself. It then banishes Father to depths of the Gate, where he had presumably come from. Truth also appears before Ed and asks what he'll give up in order to bring back Alphonse. Edward confidently declares the loss of his Alchemic powers as his toll. Truth seems a little startled by this and asks if Ed will be all right without alchemy. When Ed says he will because he still has his friends, Truth gives him a friendly smile, declaring Ed's victory over it, and fades away along with Ed's gate, rendering Ed unable to use Alchemic powers for the rest of his life.

It would appear that Truth's goal is to discourage humans from treading in God's domain when they do not deserve it, as it calls Edward a fool when the young boy sacrifices his leg to attempt human transmutation. This vision, however, collides with later encounters with the Truth during the series, in which is implied that the true goal would be to evoke growth and evolving of that alchemist through a "tough experience". Such an interpretation can explain some events such as the final talk between the Truth and Father, when Truth says that the Homunculus hasn't grown and evolved but merely leached out of others and that he "already knows" what he should have done instead. This implies that the Truth's true wish by taking the toll is to teach the Alchemist a "life lesson", a form of self-sacrifice for the sake of others, just as Edward - and, in a way, Mustang, since he only acquired the Philosopher's Stone to restore his sight after compromising himself with rebuilding Ishval - had done.

Trivia

 * In a late interview with the author, Arakawa said the Truth was somewhat a 'hollow' version of oneself (as a sort of 'internal God', or conscience), a sort of 'negative' of that alchemist, which completed itself with the tolls taken by the alchemist upon seeing the Truth.
 * The Truth shares the same English voice actress as the 2003 anime's version of Wrath, who was also responsible for taking Ed's limbs in that series. In the Japanese version, however, is depicted as having the voice of whoever encounters him with a slight effect added to differentiate, which is more in line to its role as a literal 'part' of whomever it is engaging with.
 * In the English dub, when Ed offers up his alchemy to Truth in order to bring Al back, Truth takes on a legion-like crossover of Ed and Al's voices in its shout of approval in a similar vein to the Japanese version, likely for emphasis of the scene.
 * For some reason, in the 5th Fullmetal Alchemist: Brotherhood Opening "Rain", Winry appears as Truth. This is in theory because Winry is what Ed needed most at the time of the episode in which the opening first aired.
 * Another possible reason for Winry's appearance in the opening "Rain":  at this point, the scene is playing a review of Edward's review of his journey to this point.  When he faces Truth originally, and is then dragged backward through his Door (his ten year old self), he glimpses an outline of Trisha, the object of his sacrifice that brought him to the Portal of Truth, and also the most important female in his life at that time.  But in "Rain", he sees Winry instead of Trisha, the current object of his affections and, after regaining Alphonse's body, his goal.
 * A third possibility, is that Winry-as-Truth, was meant to show what he desired as a trap, fitting the next scene: of him screaming within a philosophers stone, before images of every homunculus except Greed. Winry, his desire that tempts him away form what must be done, and the homuncili, whom he must face to succeed.
 * The ethereal being of Truth can be connected to the teachings of Theosophy (see Gate Trivia for connection between the Gate and Theosophy) wherein it is believed that "religion, philosophy, science, the arts, commerce, philanthropy and among other virtues, lead people closer to "the Absolute". Planets, solar systems, galaxies, and the cosmos itself are regarded as conscious entities, fulfilling their own evolutionary paths. The spiritual units of consciousness in the Universe are the Monads, which may manifest as angels, human beings, or in various other forms. According to Blavatsky, the Monad is the reincarnating unit of the human soul, consisting of the highest of the seven constituent parts of the human soul. All beings, regardless of stature and complexity, are informed by such a Monad." From this, we can assume that, if Hiromu Arakawa intended the gate of truth to be connected to the teachings of Theosophy, then the Truth would be the Monad.
 * This can be justified by the motto of Theosophy, which is "There is no religion higher than Truth". The emblem of Theosophy also depicts the Ouroboros in it; the Homunculi are a reference to this, as they are the children of Father, who by extension came from Truth.
 * It is also interesting to note that, in the same topic of Theosophy, they teach of the root races of humanity (of which we are in the fifth, the Aryan race,) where they teach that the Aryans established "The City of the Bridge", which is said to be below the city called Shamballa. This could be where the first theatrical film idea "Conquer of Shamballa" came from. This could also be interconnected with the Theosophic ideas of The Gate of Truth, as Dietlinde Eckhart had to pass through the Gate in the 2005 film in order to get to the supposed Shamballa that was Amestris.
 * One could also make a parallel between the meaning of Truth within FMA and the role of some deities in myths, such as Hera, a Greek goddess, in myths involving heroes - which are usually sons of her husband, Zeus. Hera is usually a deity considered to be antagonistic, posing all sorts of difficulties to the hero in question, but the utmost trait associated with Hera is the one of Pedagogy, as well as Pacts and her role in most myths is associated to a sort of "testing" of the hero and his determination. Other deities associated with this symbolic trait are Juno (roman mythology) and Morrigan (Celtic mythology),
 * Another interesting relation with Hera is that of Hera's meaning in Alchemical symbolism, by representing a stage in the Alchemic development, called "Cauda Pavonis" (peacock's tail), set somewhere in the stage of distillation, representing the moment when the alchemist comes to "understand all things for what they truly are". The peacock is also Hera's symbol animal.
 * While most alchemists see a "hollow" version of themselves while in the Portal of Truth, there is at least one case where Alphonse sees something else.  After Al works with May to offer his soul in exchange for Ed's real arm, Al re-enters the Portal to rejoin with his human body.  But the version of Truth he sees has Ed's arm, which disappears and is replaced by a "hollow" version, and also has Ed's human left leg.  This may be because Ed and Al are still "joined" through their original transmutation where they attempted to bring Trisha back from the dead.
 * The Truth appeared in an extra in volume 18 of the manga where it showed Yoki attempting human transmutation and having his hair taken as a toll.
 * In the 2013 anime-styled web series RWBY, The Truth appears as an easter egg in the first episode on the cover of a magazine (also titled 'Truth').