Approximately four centuries old in soul, forty years old in his current body, "Hohenheim of Light" remains behind the scenes throughout most of the series. Most of his past is kept under wraps, save for a few key points, revealed quite late in the series. Hohenheim is a tall, rather well-built man, standing in at at least six feet tall at first glance. He's rather gentle and weary, and soft-spoken as well, which is most likely due to the extensive amount of time spent alive, under his assorted burdens. This disposition is clearly demonstrated when Edward actually punches him in the face and kicks him in the stomach, with very little reaction.
In the anime, Hohenheim's official debut is in Episode 43: The Stray Dog (2003 series). Here, he meets Winry Rockbell, confusing her for a woman named Sara. Winry promptly flees, but soon realizes that since Sara was her mother's name she may have been mistaken for her mother. Hohenheim then appears at the Rockbell residence, where Pinako introduces him as Ed and Al's father.
Later, after Ed and Al return to Resembool, Ed reacts with fury at seeing Hohenheim flirting with Maria Ross, and he more or less declares that neither he nor Al had any time for their errant parent. But Al chooses to leave the house with Hohenheim, hoping to reconnect with his father.
His past is fairly murky, save for a few key moments shown in the series. Approximately four hundred years prior to the series, Hohenheim created the Philosopher's Stone, using people captured in a witch hunt and those dying of the plague. The resulting reaction nearly caused Hohenheim's death, but out of love and concern for her lover, Dante used the stone to attach his soul to another man's body. It's from here on out that the two jump from body to body, living on, and obtaining a so-called eternal life, although Hohenheim had only intended to live as long as the stone lasted, alongside Dante. It's not known if Dante and Hohenheim were actually ever married, but their relationship produced one son who died at the age of 18 of mercury poisoning. Hohenheim performed a human transmutation on his son, which, in turn, failed, producing the first-ever Homunculus, Envy. Regretting what he had done to his son, Hohenheim fled (although he left Dante some of the Stone). Later, Hohenheim renounces ever loving Dante, stating Trisha Elric as the only woman he ever truly loved.
Where Hohenheim was in the ten-year gap between his leaving the Elrics and turning up again isn't exactly clear. He had explained to Trisha shortly before his departure that he was leaving to do research of some kind, but never came back. In Episode 50: Death (2003 series), it is revealed that Hohenheim's stolen bodies rot more quickly every time he takes a new one. Also knowing that Dante was out there, somewhere, he assumed that staying in hiding would, perhaps, make her vanish for good. But once he found out that Dante had her eyes set on his sons, he emerged to face her, only to be immobilized by the Homunculus Sloth, who bore a striking resemblance to his late wife Trisha, and was consequently sent into the Gate, where his mind, body, and soul were separated. He was able to reassemble himself, however, and emerge on the other side, in London, and eventually becomes an adviser to Winston Churchill. When Edward's soul was sent there by Dante, he learns about Hohenheim's past. When he offers to find a way to bring Hohenheim back to their homeworld, he declines, revealing that the act is impossible due to his entire being on this side. At the end of the series, he is living in Germany, siding himself with the Thule Society, hoping to find a way to send his son back to Amestris, and caring for him until Edward left for Transylvania to find the means of world travel.
In the movie which concludes the 2003 anime, Hohenheim doesn't appear much, save for a couple of pivotal scenes. Ed explains to Noah that his artificial limbs had been created by Hohenheim, and there are several replacements kept in crates in Edward's room. Later, Ed is shocked to learn that Hohenheim is a prisoner of the Thule Society and being used as a sacrifice on Germany's side of the Gate. Envy had been captured earlier by the Thule Society, although he is stuck in the form of a giant flying serpent (the last change of his body that Envy made while traveling through the Gate to reach Earth). Hohenheim is pinned in Envy's mouth, as this is the only thing that keeps Envy placid for their mutual captors, and Envy is held above a large transmutation circle with several spears and glowing runes keeping him in place. Hohenheim tells Ed that he is a sinner, from creating Envy to the making of the Philosopher's Stone, destroying thousands of lives in the process indirectly. Ed says that none of this matters, not even getting back to Al, and says he'll free his father. But Ed realizes what Hohenheim intends, and begins to beg him to stop. Despite Ed's pleas not to, Hohenheim tells him to give Al his regards as he uses Envy to create a gate to Amestris. Hohenheim commits suicide by pulling Envy's teeth to bite him down, using his own blood and the homunculus itself to activate the Gate.