Truth

真理

Truth is the name most frequently applied to the enigmatic, ethereal entity that waits before The Gate and speaks with alchemists who have entered God's domain via alchemy. It manifests as a pure white, nearly shapeless figure, almost indistinguishable from the boundless white void surrounding the Gate, except for a shadowy silhouette that causes Truth to mirror the form of the individual it is addressing, thus reinforcing its assertion that it is merely another shade of that person. When Edward and Alphonse Elric each stood before the Gate, Truth assumed the appearance of a young boy; when Izumi Curtis appeared, Truth reflected the form of a young woman; it appeared as a vigorous man to Roy Mustang, and as a white version of Father's flask form.
Truth initially introduces itself as the entity some refer to as God, the World, or the Universe, appearing welcoming and jovial to its visitors at first before highlighting their hubris and opening the Gate, which forces them inside. At this moment, Truth often smiles with an eerily wide, insane grin resembling that of the Homunculus Gluttony and Father.
When the alchemist in question returns from their passage through the Gate, Truth informs them of the required toll for accessing the Universe's knowledge and collects it, typically in the form of a body part that disintegrates from the alchemist and reappears on Truth's form. In this manner, Truth is responsible for taking Edward's left leg and right arm, some of Izumi's internal organs, Roy's eyesight, and Alphonse's entire body.
It seems that Truth's goal is to deter humans from treading in God's domain, as it calls Edward a fool when the young boy returns and sacrifices his arm to recover his brother's soul.