Petyr Baelish
Littlefinger: a lord of small birth and enormous ambition who rose from nothing to Master of Coin, financing the crown and his own schemes in equal measure. He knew Catelyn Tully as a boy and never forgot her.
A soul born at The Fingers, sworn to House Baelish.
The fuller tale
Season One
Master of Coin, owner of half the city's brothels, and a man who has spent twenty years turning an old humiliation at Riverrun into the most useful information network in Westeros. He is helpful to Ned Stark in exactly the ways that serve him and no others, offering the gold cloaks' loyalty as a way into Ned's confidence, then selling that same confidence to Cersei the instant it's worth more to her than to him. He watches Ned die from the crowd, which is where he prefers to watch everything.
Season Two
Still Master of Coin and still trading in secrets nobody else can obtain, threading himself between Cersei, Tyrion and the Tyrells without committing to any of them further than the next piece of leverage requires. He needles Tyrion about the danger of assumptions in a room full of them, courts Margaery's family on the crown's behalf, and lets his interest in Sansa Stark show just enough to be noticed and not enough to be stopped. He survives Blackwater by simply not being there for it, having made himself useful to whichever side won before the battle was even fought.
Season Three
Brokers a Tyrell-Stark match that would keep Sansa out of Joffrey's reach and out of his own, then quietly ensures it never happens, because he has other plans for her entirely. He leaves King's Landing for the Eyrie to marry Lysa Arryn, a woman he has kept on a string for years with letters and unmet promises, and takes the Vale's power for himself the moment the vows are said. On his way out of the capital he stops to have a quiet word with Olenna Tyrell — the kind of conversation whose purpose only becomes clear much later.
Season Four
Reappears in King's Landing for Joffrey's wedding and leaves it a killer, if not by his own hand then by his own design. In the chaos after the boy king dies he spirits Sansa out of the city by ship, cloak and all, and carries her to the Eyrie as his ward. When Lysa, jealous and unraveling, threatens Sansa at the Moon Door, Littlefinger talks his wife right up to the edge of it and pushes her through, then tells the Vale's assembled lords a story about grief and suicide that not one of them dares question.
Season Five
Lord Protector of the Vale in all but name, and increasingly of the North too — he marries Sansa to Ramsay Bolton to buy her a claim on Winterfell and himself a foothold in the war to come, and leaves her there with a promise he can't actually keep, that he'll come back for her if it goes wrong. It goes wrong. He spends the rest of the season away in the Vale managing lords who'd rather not fight anyone, while the marriage he arranged turns into something considerably worse than politics.
Season Six
Brings the knights of the Vale north on his own initiative, a private army he's been building for exactly this kind of leverage, and arrives at the Battle of the Bastards late enough to look prudent and early enough to matter — his cavalry breaks Ramsay's line after Jon's forces have already paid for it in bodies. He tells Sansa, alone, that he pictures the two of them ruling from the Iron Throne together, and she lets him say it without agreeing to a word of it. By season's end she is Lady of Winterfell, and he is still working out whether that was his plan or hers.
Season Seven
Spends the season doing what he's always done — planting doubt, this time between two sisters who grew up distrusting each other for reasons that have nothing to do with him and everything to do with how useful that distrust is to him. He needles Arya about an old letter Sansa once wrote, and needles Sansa about Arya's list, certain that suspicion between them keeps him necessary to both. Sansa and Arya compare notes instead of knives, lay the pieces of his game out in front of Winterfell's whole household, and produce, as the final exhibit, a Valyrian steel dagger he lost in a bet years ago — the one that was used to try to kill Bran. Arya cuts his throat before he finishes asking for a chance to explain.
How it ends for them
On the screen. Judged and executed at Winterfell (alive in the books) (S7·E7)
The man who started the whole game — Jon Arryn's poison, the dagger, the letter — was tried in Winterfell's hall by the sisters he'd spent years playing against each other. Sansa passed the sentence; Arya's blade crossed his throat mid-plea.
The show ends him in the hall of the family he spent a lifetime dismantling. At Winterfell, believing his sister-against-sister campaign is working, he attends what he thinks is Arya Stark's trial — until Sansa, presiding, turns the charges on him: murder, and treason. The indictment is the series' full accounting, delivered by his victims' daughters with the realm's memory sitting witness — Lysa's poisoning of Jon Arryn at his word, the dagger, the betrayal of Ned, the sale of Sansa — with Bran supplying the details no living witness could. Littlefinger cycles through every tool that ever worked: denial, bargaining, a plea to Sansa's history with him, and finally, on his knees, begging. Sansa passes the sentence — she is a slow learner, she tells him, but she learns — and Arya opens his throat with the Valyrian dagger that began his game, mid-plea. In the published books he is alive, unsuspected, and winning: Lord Protector of the Vale with the realm's most valuable heiress on his arm and his fingerprints still invisible on every body. His show death is the series' promise about how his story must end; the books have yet to sign it.
Their chapters, book by book
Where the page and the screen part
The show's Baelish is the books' Baelish with the mask half off. On the page he is charm all the way down — a small, quick, endlessly amusing man whom nobody believes dangerous, which is the entire trick; readers learn his depths only when point-of-view characters fall through them. Aidan Gillen's version telegraphs menace from the first season, and the show compensates by handing him scenes the books never staged: the chaos-ladder monologue, the direct confessions to Sansa and Ros, the open sparring with Varys across the throne room. The largest divergence is structural — the marriage of Sansa to Ramsay Bolton is pure invention, and book readers note it requires show-Littlefinger to do the one thing his book self never does: surrender his most valuable asset into hands he cannot control, uninformed.
In the published novels his game is still very much afoot. He rules the Vale as Lord Protector, keeps Sansa hidden in plain sight as his bastard daughter Alayne, and is maneuvering her toward a marriage with the Vale's heir, Harrold Hardyng — a plan whose endpoint, revealing Sansa Stark to claim both North and Vale at a stroke, the show compressed and discarded. Book readers therefore know a Littlefinger who has not yet made a single fatal mistake.
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.