Dickon Tarly
Samwell's younger brother, everything their father wanted in a son: strong, martial, and heir to Horn Hill in Sam's place.
The fuller tale
Dickon is the son the doctrine ordered: strong, straight, martial, heir to Horn Hill from the moment his elder brother's inheritance was reassigned by ultimatum. The books keep him a boy glimpsed in Sam's memories — the golden younger son on whom their father's approval falls like weather, through no fault Dickon ever committed; the show ages him into a young lord riding to his first war, and gives him the trait nobody expected from Randyll Tarly's chosen heir: honesty about what he sees. The sack of Highgarden sits badly with him — men he'd hunted beside, cut down over a quarrel of queens — and he says so, to his father's visible disgust.
That honesty is the whole character, and the show spends it in one place. Offered his life on the field of the Goldroad for the price of a knee his father refuses to bend, Dickon — with rank, youth, and every excuse available to him — steps forward instead. It reads as the doctrine's one unqualified success and its final indictment in the same gesture: Randyll Tarly raised a son who would rather burn than kneel, and then stood beside him while it happened.
The page and the screen
In the books Dickon is years younger and entirely offstage — a name in Sam's chapters, a betrothal in House Tarly's ledgers, the heir-presumptive of a house whose lord is alive and scheming in King's Landing. The Goldroad field, the choice, and the fire are all show; the books have burned no Tarlys. The show also flattered him upward in age and agency to make the sacrifice legible — book Dickon is a child who has never seen a battlefield, which would have made the same scene an atrocity rather than a choice.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The show: Fell after the Reach campaign (S7·E5)
The dutiful heir of Horn Hill stepped forward to share his father's sentence, to Randyll's pride and Sam's grief. Dracarys took the future of House Tarly along with its past.
Show-only, and chosen with open eyes. After the Goldroad rout, Daenerys offers the captured survivors their lives for fealty; Randyll refuses and is sentenced to burn. Dickon announces that his father does not stand alone — over Randyll's single, startled attempt to order him back into the ranks of the living — and refuses the knee in his turn. Tyrion pleads the house's future; Daenerys declines to make exceptions; Drogon executes them together, and House Tarly's past and future go up in one column of flame while its true heir, a continent away, shelves books at the Citadel. In the published novels Dickon is alive at Horn Hill — a boy with a Valyrian sword in his future and no dragons anywhere near him.