Knight of the Laughing Tree
The Knight of the Laughing Tree was a mystery knight who fought at the Tourney at Harrenhal. He defended the honor of Howland Reed by challenging and defeating three knights whose squires had bullied him, demanding that they chastise the squires in order to ransom back their horses and armor.
The Knight of the Laughing Tree is so called because of the blazon on his shield, a smiling heart tree. His true identity remains unknown.
Most of what is known about the Knight of the Laughing Tree comes from a story told by Meera Reed to Bran Stark. As an introduction to her story, Meera suggests that this knight "might have been a crannogman". And Jojen Reed adds "or not". Jojen expects Bran to have heard this tale "a hundred times" from his father, but Bran denies any knowledge of the tale. He later explains that Old Nan was the one narrating stories to him, not his father. 
According to Meera's tale, a crannogman - most likely her own father, Howland Reed - attended the Tourney at Harrenhal. Finding there Aerys II Targaryen ("the king") and Rhaegar Targaryen ("the dragon prince"). With the Targaryens were the Kingsguard ("the White Swords"), there to welcome Jaime Lannister ("a new brother") into their ranks. Based on their known composition of the time, the knights present would be Lord Commander Gerold Hightower, Jonothor Darry, Arthur Dayne, Lewyn Martell, Barristan Selmy, and Oswell Whent.  
Among the Lords present in the tourney were Robert Baratheon ("the storm lord") and Mace Tyrell ("the rose lord"). Notably absent was Tywin Lannister ("the great lion of the rock"). Meera reports that Tywin had quarelled with Aerys and intentionally stayed away. But many of the knights and bannermen of House Lannister did attend the Tourney.   Among the women present were Elia Martell ("the wife of the dragon prince") and a dozen of her lady companions.   House Whent hosted the Tourney . A daughter of the House reigned as Queen of Love and Beauty at the opening of the tourney, though her title had to be defended against challengers. Her champions were her four brothers and famous uncle Oswell Whent. 
The crannogman found himself bullied by three squires, none older that fifteen-years-old. He had already "grown to manhood", however the boys were all larger than him in size. His only offensive weapon, a threepronged spear, was easily snatched away by his opponents. He was then thrown into the ground. His assailants repeatedly mocked him, shoved him, and kicked him. Help arrived in the person of Lyanna Stark ("the she-wolf"), who shouted indignantly that Reed was her father's bannerman. She beat off his attackers with a tourney sword, scattering them away.  
Reed was wounded and Lyanna took him back to her "lair", probably a tent. She cleaned his wounds and bound them with linen. Then introduced the guest to her brothers Brandon Stark ("the wild wolf"), Eddard Stark ("the quiet wolf"), and Benjen Stark ("the pup"). That evening, Lyanna persuaded Reed to attend the feast marking the start of the tournament. She insisted that he was highborn and had as much right to attend as anyone. The "wolf maid" was not easy to refuse. Benjen found a suitable garb for Reed. Within Harrenhal, Reed ate and drank with the Starks. Their table also included certain Stark sworn swords, notably members of Houses Dustin ("barrowdown men", men from the Barrowlands or Barrowton), Hornwood ("moose"), Mormont ("bears"), and Manderly ("mermen").  
During the feast Prince Rhaegar Targaryen, a noted musician, performed a sad and beautiful song that made Lyanna weep; when Benjen teased her for crying, she poured wine over his head. A recruiting officer of the Night's Watch was there, trying to convince knights to join the Watch. Robert Baratheon was involved in a drinking contest with Richard Lonmouth ("the knight of skulls and kisses"). Meera notes in passing that Robert won the contest. Reed focused his attention on Ashara Dayne ("a maid with laughing purple eyes"), a companion of Elia, who was dancing in turns with various partners. First a member of the Kingsguard (either her brother Arthur Dayne or infatuated Barristan Selmy), secondly Oberyn Martell ("a red snake"), third Jon Connington ("the lord of griffins"), and lastly Eddard Stark. Eddard was too shy to ask her for a dance, so Brandon asked her on behalf of his brother.   
Still during the feast, both Reed and Lyanna recognized the three bullying squires. One serving a pitchfork knight (House Haigh), one serving a porcupine (House Blount), and the last boy serving a knight of two towers (House Frey). She pointed them out to her brothers. Benjen offered to find Reed a horse and armor in order to avenge himself. Reed failed to reply, torn with indecision. His pride demanded vengeance. But he was afraid of loosing and making a fool of himself. Bringing shame to his people in the process. He was no knight, not used to horses and lances. Before going to sleep, Reed prayed to the Old Gods. Then went to sleep at the tent of Eddard, as the "quiet wolf" had offered him hospitality.  
During the first two days of the tournament, the porcupine knight, pitchfork knight, and the knight of the two towers each won a place among the the champions. All in jousting. Late on the afternoon of the second day, a mystery knight appeared in the lists. He was "short of stature". His armor was made up of mismatched armor bit and pieces, and appeared and ill-fitting on him. His shield blazoned with the image of "a white weirwood with a laughing red face". The tale earlier mentions that Reed had with him "a leathern shield" but does not specify whether this was the same shield. He challenged and defeated all three of the previously mentioned knights. Winning custody over their horses and armor. None of them were particularly popular, so the smallfolk cheered for his victory. When the defeated trio sought to ransom back their former property, the Knight declared his terms. That they ought to teach their squires honor. His voice sounded "booming" through his helm. The trio proceeded to chastise their squires sharply. 
The Knight attracted unwanted attention. Robert Baratheon and Richard Lonmouth were determined to unmask him, while Aerys II was certain that the man was his enemy. By the next morning, the Knight had disappeared. An angry Aerys send Rhaegar to search for the vanished Knight. But only the shield of the Knight could be found, abandoned in a tree. 
- The mystery knight must have been someone who knew about Howland Reed's experience with the squires and was motivated to avenge him. For the fans this points to the Stark siblings and Howland Reed self as the books do not mention that any other person knew the story.
- His height was below average, or else it would not be noted in the first place.
- The mismatched armor would point to the Knight not having access to regular armor, or having regular armor with tell-tale markings on it.
- The booming voice was most likely not the regular voice of the Knight.
- The weirwood image implies a connection to the Old Gods and the North. 
- He was competent enough at the joust to defeat three knights who had made it through the first day of the tourney, but as none of them were particularly well-known, he need not have been unusually skilled.
- Meera was surprised that Bran had never heard the story before, which suggests that the Knight may have been either a Stark, or someone close to the family. 
References and Notes
- A Storm of Swords, Chapter 24, Bran II.
- "The Citadel: FAQ, entry 6.2.6- Who were the different people in Meera's story?" (2007
- A Dance with Dragons, Chapter 67, The Kingbreaker.
- Conversation topic: The Knight of the Laughing Tree on Westeros Forum
- Conversation topic: Howland Reed=Knight of the Laughing Tree? on Westeros Forum
- "The Citadel: FAQ, entry 6.2.7- Who was the Knight of the Laughing Tree?" (2011)
- Conversation topic: Knight of the Laughing Tree on Westeros Forum
- Conversation topic: Who was the Knight of the Laughing Tree? on asoiaf.yuku.com