Truth is the unknown being that watches over the Doorway to Truth. The one who collects the toll of all those that look inside.
Truth is an ethereal being of unknown origin. It's from the Fullmetal Alchemist series created by Hiromu Arakawa. It sits at the Doorway to Truth and collects the toll from all who enter it's realm and wish to see inside the doorway upon attempting human transmutation through alchemy. Truth is an empty representation of one's own self. The form of an internal god. It takes on the very size and shape of the person who sees it.
The price of looking into the gate for the truth is a deeply ironic one. For each of the characters in the Fullmetal Alchemist series that saw inside the gate. Truth took from them what they wished for the most.
Edward Elric wanted to stand at the head of the family. In the attempt to resurrect their mother, Trisha Elric, through human transmutation. What Truth took from Ed was his leg so he could no longer stand. When Ed used this technique again to rescue his brother's soul from the other side, Truth took the arm of Ed that he used to try and hold Al as he was taken away.
Alphonse Elric assisted his brother in what was the failed attempt to resurrect their mother. What he had wish for most was to feel the warm of their mother again. For that desire, Truth took Alphonse's entire body, and his soul was placed into the body that they boys had created in their attempt that human transmutation. When Edward pulled Al's soul back. It was then placed into an iron-armor body that could not feel cold or warmth.
Izumi Curtis is the alchemy teacher of the Elric brothers. She had lost her unborn child due to an illness. She attempted to resurrect her child. The toll Truth took was may of her internal organs so that she could never again conceive a child.
Roy Mustang was not trying to bring anyone back from the dead and he was forced into opening the Doorway. Still, what Roy had wanted most always had him looking to the future of his country in order to make it better. The toll Truth took from Roy was his eyesight.
The voice actors vary for Truth among the different translations. For the Japanese versions, Truth is voiced by the actor of what character is currently speaking with it. As an example, if Edward Elric is talking with Truth, then the character is voiced by his Japanese voice actress, Romi Park. For the English release, Truth is given a single voice actress of Luci Christian.