Francis Crawford of Lymond

Overview
Francis Crawford of Lymond is the lead character of Dorothy Dunnett's series of novels, known as the Lymond Chronicles.

Biography at the time of Game of Kings
Francis Crawford of Lymond was born in November 1526, the second son of Sybilla (Semple) Crawford, and of her husband, Baron Culter. He has an elder brother, Richard Crawford. When Game of Kings opens in 1547, Richard has inherited the family Barony and is the Third Baron Culter. When the Second Baron died, RIchard became the Third Baron Culter, and Lymond inherited Richard's erstwhile courtesy title The Master of Culter. However, because Lymond was outlawed he no longer legally holds this title (although he is referred to as "The Master of Culter" throughout Game of Kings). Lymond's grandfather, the First Baron Culter, was a charismatic figure who died about three years after Lymond's birth.

Backstory to The Game of Kings (spoilers for TGOK):

In 1542, the English invaded and defeated the Scots at the battle of Solway Moss. Francis Crawford, and many other Scots soldiers and nobles, was captured and taken to the court of Henry VIII. There he met Margaret Douglas, and she seduced him. He wrote a letter to be delivered to his friends and family in Scotland, but it was intercepted and text added to make it seem as if he were writing to English colleagues. English agents took the letter to Scotland and blew up the convent at Lymond, killing Lymond's sister Eloise and a number of nuns. The letter was left at the scene, implicating Lymond in both the murders and as a long-time spy of the English.

Upon his exposure as an English spy and traitor to his country, Lymond was moved from London to Calais, where he was captured by the French and made to serve as a galley-slave for two years. At the end of that time, he was discovered by Lord Lennox, who freed him. The exact sequence of events that followed is unclear, but at one point Lymond was fighting under the command of Lord Wharton, and purposefully bungled several missions, to Wharton's embarrassment. Lymond may have re-started his affair with Margaret Douglas, Lord Lennox's wife, during this time. Eventually he left England altogether and returned to the Continent, where he drew together the mercenary company with whom he returned to Scotland in 1547, where The Game of Kings begins.

Family History (spoilers for the entire series):

Francis is some 10 years younger than is brother, and relations between them are not easy. His sister, Eloise, died in an explosion in the ruins of a convent some years earlier when she was 13. Lymond was implicated as part of an espionage plot involving Henry VIII. He is close to his mother. He was disliked by Gavin Crawford, Second Baron Culter, and the dislike was returned.

The true history of Francis Crawford's birth is the major story-arc of the series, at the end of which it is revealed that Sybilla Semple, in France as a young woman, met and secretly married Francis Crawford, the First Baron Culter. He was lost at sea, and presumed dead. Upon her return to Scotland, she married Crawford's son, Gavin, to whom she bore a son Richard. Several years into her second marriage, which proved to be very unhappy, her first husband,   Francis Crawford, the First Baron, returned to Scotland. All three parties swore to keep the tangle of relationships secret, and by this  agreement  Sybilla resumed her love relationship with the First Baron, while staying married to Gavin in name only. To the First Baron she bore Francis and, a few years later, Eloise, whom the world thought to be Gavin's.

This explains the Second Baron's dislike for his "son" Francis, as Francis was, in fact, his half-brother through his own wife. But because Sybilla's marriage to Francis Crawford predated her marriage to Gavin, her younger son Francis Crawford of Lymond was actually, technically, the true heir to the title of Culter. By the time this is revealed, however, Lymond and Richard have a sufficiently strong relationship that Lymond chooses not to publicize the true story, and leaves Richard and his heirs in possession of the title.