Father is the creator of the Homunculus. An evil copy of Van Hohenheim, he's a creature who aims to become the perfect being.
During the times of the ancient civilization Xerxes, Father was once a small shadow-like creature in a flask that was born from an unknown alchemist experiment that used the blood of a young slave known only as Number 23. The creation asked to be called " Homunculus" and out of gratitude for the blood that brought him into the world. The creature then named the slave "Van Hohenheim". Homunculus had a great amount of knowledge about alchemy, which he had taught to Hohenheim. This knowledge had helped Van rise from slave to associate alchemist to his master. Hohenheim wished for a family, but Homunculus wished for a like outside of this flask.
In order to gain his freedom, Homunculus had tricked the King of Xerxes by promising him immortality through making a transmutation circle that would surround his kingdom. The dwarf kept the details of the plot from Honeheim that the cost would be the slaughter of the innocent throughout the land of Xerxes and took several years to complete. In the end, all of the people in Xerxes had their souls ripped from their bodies to create the Philosopher's Stone. Homunculus had lied to the king about the center as it was standing at the true center with Hoheneheim.
He would awaken in a new vessel that was made to look like Honeheim and had divided the Philosopher's Stone with him to grant an immortal body. Van was horrified as he was surrounded by the bodies of his friends that surrounded them both. They both separated, Homunculus traveled to the west and Hohenheim went into the East to the land of Xing. Homunculs had planned to become the perfect being by taking the power of God, but would need to recreate the same plot he used in Xerexes, only on a far larger scale. He began by creating the small country of Amestris.and taught his form of alchemy to it's people. In his idea of perfecting himself, he separated the sins of his body into the embodiments called the Homunculi, with a philosopher stone core and they called him "Father". With these powerful creatures as his own family, they would create an army and manipulate the world to build the country and the transmutation circle for Father's plot.