Leo is the son of Hephaestus and the mechanic of the Argo II (a ship capable of sea and land travel, not a sequel to a hit Ben Affleck flick). Leo somehow manages to have both the simplest and the most complex motivation. What a bundle of demigod contradictions.
His main goal is to return to Calypso, a girl he fell in love with on the island of Ogygia, "an island that no man ever found twice (9.61).
Returning to Calypso is particularly important to him because his six other friends are all paired off romantically: Percy & Annabeth, Frank & Hazel, and Piper & Jason are all K-I-S-S- I-N-G. But Leo was told that he "will always be the seventh wheel" (9.60). (Which, according to Shmoop, ain't all that bad—but we don't make the rules.)
Despite being lonely, Leo is a crucial member of the team as they search for something called the physician's cure. Examples? His gadgets help defeat the goddess Nike, and he reprograms a maniacal statue of Hygeia to defeat itself.
But why do they need the cure to begin with? Well, there's a prophecy saying one of them will die. Here's the thing, though: Leo wants to be the one to die, so he hides the cure once they get it. And sure enough, when Gaea is resurrected, Leo destroys her in a big explosion…and dies. Everyone mourns his loss.
At least, that's what everyone else thinks.
In the final chapter, we learn that Leo somehow rigged up a device to inject himself with the cure…and he lives. Plus, there turns out to be some sort of loophole when it comes to Calypso. If a man can only find her island once in his lifetime, well, he gets a second shot when he's reborn. Leo swoops down to retrieve Calypso, and together they soar "into the unknown" (58.58).
Cue romantic music.