He was the ferryman in the old myths. He navigated a boat... to the Underworld.
Charon is a minor character on Once Upon a Time. He is the ferryman who drags souls to the Underworld, most notably arriving from a lake in Storybrooke, coming to claim Robin Hood's life when met with a Fury, then again to take all of the heroes when they're marked by past Dark Ones, and finally when the heroes team up to travel to the Underworld in order to save Captain Hook.
After the Third Curse
When the moon reaches its peak, the ferry from the Underworld will arrive... and drag us down there.
Charon comes to claim Robin's life. ("The Price")
The Fury drops Robin by the side of a lake so it can summon a portal to the Underworld. Mist magically appears on the water, and out of it appears a boat with a hooded figure on it. However, Snow, Charming, Grumpy and Arthur arrive, hoping to stop the monster. Instead they're knocked to the floor by a magical pulse, allowing it time to suck Robin's life force away. However, Regina arrives and runs towards the Fury, offering her life in place of Robin's. The Fury changes its target to Regina and begins sucking away her life force, causing a worried Snow to ask what she's doing, "What I should've done in Camelot" she replies. Snow rushes to Regina's side and tells her she doesn't have to do this alone, "I'm with you" she says before grabbing Regina's hand, causing the demon to start sucking her life force too. Charming joins in too, and then he's soon followed by Grumpy and Arthur. The demon starts sucking all five of their life forces at once, but it becomes too powerful for it to control and its magic ends up exploding back on itself, sending it flying away. The portal to the Underworld proceeds to close and the hooded figure on the boat vanishes. ("The Price")
The Charon has the Dark Ones as his passengers. ("Broken Heart")
After dueling Rumplestiltskin, Hook, having remembered that he is a Dark One, now has what he needs in order to open a portal to the Underworld: the blood of a man who's been to hell and back. He dips his hook, which is doused in Gold's blood, into a pond in Storybrooke, causing a lot of mist to appear. A boat soon emerges from this mist, led by the Charon, and a myriad of cloaked figures are his passengers. Every single past Dark One, returning to the land of the living. Nimue, in her mask, is the first to exit the boat and approach land. Hook greets her as she reveals her green and scaly face, and she tells him that they're all there as promised, in the flesh. She adds that now it's time for them to get to work and do what Dark Ones do best, which is snuff out the light. Hook welcomes her to town, excited for this prospect. ("Broken Heart")
Emma and the others are voluntarily headed to the Underworld. ("Swan Song")
With the Dark Ones having been unleashed within Storybrooke, they mark living victims to take their places in the Underworld so that they may return permanently to the land of the living. Those emblazoned with the Mark of Charon include Regina, Robin, Snow, Charming, six of the seven dwarfs, Henry and Mr. Gold. When the moon reaches its peak, the Charon comes to take everyone away; however, Hook has a sudden change of heart and absorbs all the Dark Ones into Excalibur, then making Emma strike him down with it. This means that no living sacrifices are needed and that the darkness is returned to Hell where it belongs... but so is Hook. And when Emma learns that the darkness didn't even die, instead having redirected into Rumplestiltskin thanks to his meddling, she wants to go to the Underworld to save Killian. Snow, Charming, Regina, Robin and Henry tag along, and Gold drips his blood into the town pond in order to summon the Charon. His ferry arrives from the mist and everyone walks across the water to get a space on board, with Emma vowing to find her lost love. ("Swan Song")
The heroes arrive at their destination. ("Souls of the Departed")
Emma's eyes spring open on the ferryboat Underworld. She seems to have passed out, to the concern of her parents, and they ask her if she's okay as she continues to breathe heavily following her otherworldly encounter with Neal. Despite how she seems, she assures them both that she's fine, and Snow and Charming each have a hand on their daughter's shoulder as a sign of comfort. "Good," says Rumple in response to the savior's proclamation, "Because we're here." He is at the head of the boat, watching beside Charon as they approach the entrance to the Underworld. Blue mist continues to surround them on the murky waters, and the others who are aboard seem as determined as ever, though also more cautious than usual as they delve into their next great adventure. ("Souls of the Departed")