Mycah, the butcher's boy
A butcher's boy travelling with the king's party, who befriends a Stark girl and plays at swords with her by the river.
The fuller tale
Mycah is in the story for a handful of scenes and underneath it forever. A butcher's son attached to the royal party's provisioning on the long road south from Winterfell, he is perhaps twelve, red-haired and freckled in the books' single description, and possessed of the one qualification that matters to the story: he is willing to play swords with a highborn girl when every other child on the kingsroad knows better. He and Arya Stark spar with broomsticks along the Trident's banks — her first sparring partner, before Syrio, before Needle — and for an afternoon the saga permits itself the sight of two children playing at the thing the adults around them do in earnest.
The game is interrupted by the crown prince, and everything that follows is the realm in miniature. Joffrey, drunk on wine and rank, decides the peasant boy has struck a noblewoman and sets his sword to Mycah's cheek; Arya's defense of her friend — the broomstick, Nymeria's bite, Joffrey's humiliation — becomes a crisis of state in which every adult understands the truth and only one child pays for it. Mycah runs, because a butcher's boy accused by a prince has no other move on the board. The story never lets his smallness be forgotten: he is the first entirely innocent casualty of the game of thrones, the proof — delivered before the wars even start — that the realm's ledgers bill the powerless for the sins of the powerful, and his name stays in the tale for seven seasons because Arya Stark refuses to stop saying it.
The page and the screen
Page and screen tell his short story almost identically — the Trident, the broomstick, the flight, the Hound — and differ only in what they do with the echo. The books make him a permanent stone in two consciences: his name is a fixture of Arya's nightly death-prayer from the first, and his memory becomes the unlikely hinge of Sandor Clegane's book arc — flung at the Hound as an accusation at his trial by the Brotherhood Without Banners, and carried by him, the story implies, a good deal further than he admits. The show keeps the list but stages the reckoning differently, giving Arya and the Hound seasons of road together in which the butcher's boy is the unspoken third traveler. In both tellings his function is identical and terrible: he is the measure by which we are meant to judge everyone who was on the kingsroad that day — and by that measure, nearly all of them are found wanting.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The show: Ridden down on the kingsroad (S1·E2)
The books: Killed on the kingsroad (AGOT, ch. 17)
Arya's friend played at swords with a princess by the Trident and paid a prince's grudge for it. The Hound rode him down on the kingsroad; his father was given the body.
He is ridden down on the kingsroad, and no version of the story pretends it was anything but murder. After the incident at the Trident, with Joffrey humiliated and the court requiring blood from somewhere, Sandor Clegane is sent after the fled boy — on whose order the tellings leave technically unspoken, though Cersei's appetite and Joffrey's lie are the only candidates — and the Hound does what the Hound is for: he runs Mycah down on horseback and cuts him nearly in half. The books deliver the detail through Ned Stark's horrified viewing of the body, returned to the butcher his father like spoiled goods; the show gives it a single image, the small form slung across the Hound's saddle, and the line that damns and defines Sandor for seasons — he ran, but not very fast. It is the first death of the saga's long wars, inflicted for a prince's wounded vanity on the one person present who had done nothing at all; Arya adds the Hound to her list for it that night, and the story spends two entire character arcs — hers and Sandor's — paying off the debt incurred beside the Trident.