Question: Who was the wife of the king of the Scots who was assassinated on 21 February 1437?

Context paragraphs:
James II of Scotland: James II (16 October 1430 – 3 August 1460), who reigned as king of Scots from 1437 on, was the son of James I and Joan Beaufort.  Nothing is known of his early life, but by his first birthday his twin and only brother, Alexander, who was also the older twin, had died, thus making James the heir apparent and Duke of Rothesay.  On 21 February 1437, James I was assassinated and the six-year-old Duke of Rothesay immediately succeeded him as James II.

Isabella, Countess of Lennox: Isabella of Lennox (d.1458) was the ruler of Lennox, from 1437–1458, and last in the line of Mormaers or Native Scottish rulers.  As the wife of Murdoch Stewart, 2nd Duke of Albany (d.1425), she was also Duchess of Albany (1420–1425), but in 1425 her family would be almost completely destroyed when her husband, father and two sons were executed by the vengeful King James I of Scotland.  Only one son, James the Fat, would escape the King's wrath, and he would die in exile in Ireland soon after.  Isabella succeeded in escaping the fate of her family, and would eventually regain her title and estates, retiring to her castle in Loch Lomond where she raised her grandchildren, the children of her youngest son.  She would eventually live to see the violent death of her former persecutor, King James.  Though none of her four sons survived her, her grandson Andrew Stewart, 1st Lord Avandale would in time rise to become Lord Chancellor of Scotland.

Sigismund, Holy Roman Emperor: Sigismund of Luxemburg (15 February 1368 – 9 December 1437) was Prince-elector of Brandenburg from 1378 until 1388 and from 1411 until 1415, King of Hungary and Croatia from 1387, King of Germany from 1411, King of Bohemia from 1419, King of Italy from 1431, and Holy Roman Emperor for four years from 1433 until 1437, the last male member of the House of Luxemburg.  Sigismund von Luxembourg was the leader of the last West European Crusade - the Crusade of Nicopolis of 1396 to liberate Bulgaria and save Constantinople from the Turks.  Afterwards he founded the Dragon Order to fight the Turks.  He was regarded as highly educated, spoke several languages (among them; French, German, Czech, Hungarian, Italian, and Latin) and was an outgoing person who also took pleasure in the tournament.  Sigismund was one of the driving forces behind the Council of Constance that ended the Papal Schism, but which in the end also led to the Hussite Wars that dominated the later period of Sigismund's life.

Peter III of Russia: Peter III (21 February 1728 – 17 July [O.S. 6 July] 1762 ) (Russian: Пётр III Фëдорович, "Pyotr III Fyodorovich" ) was emperor of Russia for six months in 1762.  He was born in Kiel as Karl Peter Ulrich, the only child of Charles Frederick, Duke of Holstein-Gottorp, (the child of Hedvig Sophia of Sweden, sister of Charles XII), and Anna Petrovna, (the elder surviving daughter of Peter the Great).  The German Peter could hardly speak Russian and pursued a strongly pro-Prussian policy, which made him an unpopular leader.  He was deposed and possibly assassinated as a result of a conspiracy led by his German wife Princess Sophie Friederike Auguste von Anhalt-Zerbst-Dornburg, who succeeded him to the throne as Catherine II.  His death could also have been the result of a drunken brawl with his bodyguard when he was being held captive after Catherine's coup.

Foua Toloa: Foua Toloa (died 23 June 2015) was a Tokelauan politician who served as the Head of the Government of Tokelau, or "Ulu", from 21 February 2009, to 21 February 2010.  He was a member of the Council for the Ongoing Government of Tokelau, and was a "faipule" on Fakaofo.  As the office of Ulu rotates on an annual basis, Toloa resumed office on 21 February 2011.  For a time in 2011-2012, he was also Tokelau's minister for Finance, Telecommunication, Energy and Transport.  He died in California in 2015.

Louis of Cyprus: Louis of Savoy (Italian: "Ludovico" ; 5 June 1436 or 1 April 1437 – August 1482) was King of Cyprus, reigning together with and in the right of his wife, Charlotte.  He was the second son and namesake of Louis, Duke of Savoy, and his wife Anne, daughter of King Janus of Cyprus.  He was born in Geneva.

John Stewart, Earl of Mar (d. 1479): John Stewart, Earl of Mar and Garioch (c. 1456–c. 1479) was the youngest surviving son of James II of Scotland and Mary of Guelders.

James I of Scotland: James I (late July 1394–21 February 1437), the son of King Robert III and Annabella Drummond was the youngest of three sons.  By the time he was eight, both of his elder brothers were dead—Robert had died in infancy but David, Duke of Rothesay, died suspiciously in Falkland Palace while being detained by his uncle, Robert, Duke of Albany.  Although Albany was exonerated by parliament, fears for James's safety grew through the winter of 1405–1406 and plans were made to send him to France.  In February 1406, James was accompanying nobles close to his father when they clashed with supporters of Archibald, 4th Earl of Douglas, forcing the prince to take refuge in the castle of the Bass Rock, a small islet in the Firth of Forth.  He remained there until mid-March when he boarded a vessel bound for France, but on 22 March while off the English coast, pirates captured the ship and delivered James to Henry IV of England.  The ailing Robert III died on 4 April and the 12-year-old James, now the uncrowned King of Scots, would not regain his freedom for another 18 years.

Walter Stewart, Earl of Atholl: Walter Stewart, Earl of Atholl, Strathearn and Caithness (c. 1360 - died 26 March 1437) was a Scottish nobleman, the son of Robert II of Scotland.  Stewart was an enthusiastic advocate of the ransom and return to Scotland of the future king in exile, James I, in 1424.  In 1425 he served as a member of the jury of 21 which tried and executed his nephew Murdoch Stewart, 2nd Duke of Albany.  Eventually however Atholl turned against the King and conspired in his assassination in 1437.  He was tried for murder and was executed after 3 days of gruesome torture.

Catherine Douglas: Catherine Douglas, later Catherine "Kate" Barlass, was a historical figure who tried to prevent the assassination of King James I of Scotland on 20 February 1437.  She was a lady-in-waiting to Queen of Scotland, Joan Beaufort.

Supporting paragraphs: John Stewart, Earl of Mar (d. 1479) James II of Scotland
Answer: Mary of Guelders