Meryn Trant
A knight of the Kingsguard, a joyless brute happy to do the ugly work a cruel king requires and call it duty.
A soul born at The Crownlands, sworn to The Kingsguard.
The fuller tale
Season One
A Kingsguard under Robert and then Joffrey, and one of the few in white cloaks who never bothers dressing up his cruelty as duty. After Ned Stark is arrested he is sent with a squad of gold cloaks to seize Arya from her dancing lessons, and it is his blade that ends the standoff with Syrio Forel, who holds the stairwell with nothing but a wooden practice sword long enough for her to run. He is in the crowd at Baelor's steps when Ned is executed, one white cloak among several, doing the job the office requires without needing to be told twice.
Season Two
Joffrey finds a use for him beyond standing guard: when Robb Stark wins another battle in the field, Joffrey has Meryn Trant beat Sansa for her brother's victories, and he does it in open court without hesitation, tearing at her clothes on command until Tyrion arrives to put a stop to it. He carries out the order the way he carries out every order from Joffrey, with no visible feeling about it either way.
Season Four
Still in white at Joffrey's wedding to Margaery, positioned near the high table when the king starts choking on poisoned wine, and he does exactly as much to help as every other Kingsguard in the room, which is nothing useful. He stays on with the household after the crown passes to Tommen, one more sword nobody thinks to reassign.
Season Five
Sent to Braavos on the Small Council's business, where he spends his evenings at a theater that turns out to be a front for something much worse, buying the attention of the girls who work there. One of the newer girls, quiet and good with a blade, wins his trust over several visits before he ever learns her name is Arya Stark. She kills him in a back room, taking her time about it, and leaves the theater a small, precise settlement of a debt several seasons old.
How it ends for them
On the screen. Killed in Braavos (S5·E10)
The Kingsguard brute who killed Syrio Forel and beat Sansa on command took his appetites to a Braavosi brothel. A girl with a borrowed face put out both his eyes before she cut his throat — the first name crossed off Arya's list by her own hand.
In the show, the first name Arya Stark crosses off her list by her own hand is his. Recognizing Trant on the canals during his Iron Bank escort duty — the man who killed Syrio Forel — she abandons her assigned Faceless errand and stalks him to the brothel where his tastes have emptied the room of willing girls. The child brought to him last is a girl with a borrowed face. What follows is deliberate, prolonged and shot without mercy: she puts out both his eyes with her knife, names herself and her grievance so he dies knowing exactly whose blade it is, and opens his throat. The kill costs her — the House of Black and White exacts payment for a life taken from the wrong ledger, and Arya's blindness follows — but the show frames the debt as one she never disputes owing. In the published books Meryn Trant is alive in King's Landing, his appointment with the god of death not yet on any page.
Their chapters, book by book
Where the page and the screen part
Book Trant is the same instrument with less screen time and no Braavos. The novels use him exactly as the show does — Syrio's presumed killer, Sansa's beater, the reliable hand for shameful work — but he never leaves Westeros, and the pedophilia the show assigns him is a television invention constructed to sharpen his ending. In the published books he is alive: still a Kingsguard under Tommen, still unremarkable, last seen holding the door of the order's decline. His Braavosi excursion belongs to the show's compression of Arya's training arc, which needed a name from her list to walk within a blade's reach of a novice Faceless Man; readers of the books are still waiting to learn which telling of his story the novels will rhyme with.
Their journey
Where the two tellings send them to the same place the road runs whole; where they part, the screen keeps the left and the books the right.