House Tyrell
Wardens of the South and Lords of Highgarden, stewards risen to rule the vast, fertile Reach. Lord Leo Tyrell, called Longthorn, is reckoned one of the finest lances of the age when he deigns to ride — and liege lord over most of the tale's quarreling small houses.
The fuller tale
No Tyrell claim to Highgarden runs back past a few centuries, and the family has never quite stopped being reminded of it — they rose as stewards to the old Gardener kings, inherited the seat when their masters' line died out at the Field of Fire, and have spent every generation since governing a Reach full of houses considerably older than themselves, several of whom have never fully forgiven the promotion.
What silences most of that resentment is simple abundance: Highgarden's harvests, its chivalry, its tourneys are the grandest the Seven Kingdoms produce, and a house that controls that much plenty does not need old blood to command real loyalty, only the good sense to keep sharing it out.
The Reach's finest lance in this generation is Leo Tyrell, called Longthorn, though the qualifier that follows his name in most halls is when he deigns to ride — a man who could plausibly out-joust nearly anyone in the kingdom and mostly declines to bother proving it, which is its own kind of aristocratic confidence. As liege lord over Ashford, Osgrey, Fossoway, Peake and half the tale's other quarreling small houses, his more constant occupation is simply keeping the Reach's chronic squabbling from boiling over into anything Highgarden has to answer for.
That the era's single most consequential tourney happens on Tyrell land, among Tyrell bannermen, without a Tyrell needing to lift a lance in it, says something about how the house prefers to hold power in this generation — present at the center of things by right of who owes them fealty, rarely by need of showing up in person.
The page and the screen
Leo Tyrell is unadapted so far — no confirmed casting exists for him — and Highgarden itself has not been shown on screen; The Hedge Knight's action stays at Ashford Meadow, a Tyrell vassal's holding, without the season needing to travel to the seat of the Reach itself.