Marq Baelish was briefly Lord Paramount of the Riverlands, assuming his position after the arrest of his cousin Emmon Baelish. He was declared a traitor by King Damon Lannister in the second era and fled Harrenhal with his cousin, Alicent, where they remained in the Riverlands as outlaws briefly, before Marq was captured and executed by Edwin Frey.
As a child, Marq quickly realized he was different from the other boys and lordlings. He was quick where they were slow, and clever when they were not. He seemed to be good at most things - a fast learner, a quick reader, and he seemed to be a natural marksman with a bow.
As he grew older, taller and stronger, he became better at sword-fighting and archery. He honed his reading and writing skills and became an intelligent and sharp young man. He always dreamed of glory, of fame and fortune. His time as a squire only amplified this, and he became obsessed with dreams of the Kingsguard, the Nights Watch and famous knights. He was knighted at age 16.
When the Lord of Harrenhal and his heir were killed in terrible incidents, Marq became suspicious of Emmon Baelish's involvement. Always aware of his position, he realized his best chance of fame were to stay close to his lordly cousin, ready to fill his seat.
When Emmon Baelish was captured and arrested for the murder of James Arryn, Marq Baelish assumed the Lord Paramouncy, skipping over Alicent Baelish on account of her womanhood. He swore allegiance to Damon Lannister and promised support for his claim, but did not send his army to the Kingswood when they were called, causing him to be branded a traitor.
When the King sent Lord Aeron Greyjoy to plunder the Riverlands, Marq Baelish kept his troops garrisoned in Harrenhal, refusing to come to the aid of his vassals. Alicent attempted to warn him of the consequences, but was met with little regard. When the King's army laid siege to the castle, he fled, becoming an outlaw, and hoping to reassure his paramountcy by wedding his cousin and gathering support amongst the Riverlords still loyal.
Marq was met with little consideration from any of the lords, and was eventually captured by Edwin Frey, where he was promptly executed. His cousin, Alicent, however, managed to escape and make a life for herself in Lannisport.
"[ Damon Lannister ] set fire to the [Riverlands] and pillaged the kingdom indiscriminately. I suppose the King thought that if he lit enough fires under the mocking bird's nest, it would smoke him out." - Lord Manfrey Fowler
"The craven titan fled. No one is sure of his whereabouts, but he will certainly find no safe haven in his own kingdom. Any of his vassals would gladly hand over the man who let their homes be burned and their crops destroyed while he cowered in his castle and feasted as if he were the King of the Seven Courses himself. Hand him over, that is, if they don't kill him first." - Lord Anders Sand
Robert Baelish, uncle
Sansa Arryn, aunt
Petyr Baelish, cousin
Emmon Baelish, cousin
Alicent Baelish, cousin