Some things we know:
1. Jojen and Meera themselves when they relate the story don't know who it was. Meera thinks it was a crannogman because the Knight was shorter than normal, but Jojen says'Or not'.
2. Jaime was a newly made Kingsguard at 15in the first days of the tournament with stars in his eyes about Selmy and Arthur Dayne and honour and knighthood. Becoming a KG would change all that rapidly when he got to know Aerys, but there was no doubt at this point he was romantic and idealistic.
3. Right before the tournament, as a squire in his first real battle, he had just assisted Arthur Dayne to defeat the Smiling Knight, the greatest villain of that age. His hero is the perfect knight Arthur.
4. He wanted to stay at the tournament but was ordered away to KL. That would have been extremely hard on him after the excitement of being knighted, made a Kingsguard, at a tourney where he likely would have made his first appearance as a full knight fighting for the honour of himself or a lady.
5. As he's leaving, he hears about Lyanna's encounter with the squires or overhears Howland talking it all out at the Weirwood tree. This gives him a cause to fight for the lady's and Howland's and knighthood in general's honour.
6. He has been ordered away so can't appear as himself or in his own arms. What better than to appear as that most romantic of tournament icons, a Mystery Knight, and uphold the ideals of knighthood?
6. The arms: nobody has so far explained satisfactorily for me the Laughing Face on the weir wood tree. The weir wood is most easily explained. The Mystery Knight fights on behalf of Lyanna Stark and Howland Reed, both weir wood worshippers, and is possibly answering a prayer made to a weir wood tree. But what of the Laughing Face? Is it a coincidence that he and Arthur Dayne had just defeated the Smiling Knight, the greatest villain of the Age, and here comes a Mystery Knight wearing the emblem of a Laughing Knight. Moreover, as a key personality trait, Jaime out of everyone is said to be always laughing. What better homage to Arthur Dayne, his hero who killed the Smiling Knight, than to fight with an emblem that recalls that historic defeat and symbolizes his own personality?
7. Although he had not attained his full height at 15 and is a comparative newby, he has trained in jousting for years, and moreover is exceptionally talented. He is able to defeat the more experienced knights. And let's see here: his request is purely Jaime. He himself has just risen from squire to knight. He likely know the squires well. Why not uphold knightly honour and at the same time embarrass those layabout squires? I can see him laughing indeed.
8. Aerys, who is mad and paranoid, identifies the Mystery Knight, perfectly correctly, as his enemy and wants him found. This seemed a little odd even for Aerys. But I suspect we'll find that Aerys was prophecy-cursed and had a little foresight. Because the greatest enemy of Aerys is of course the Mystery Knight if Jaime is the Mystery Knight. It is Jaime who will murder Aerys nine years later.
9. There is all kinds of irony here. Aerys has a premonition about the Mystery Knight but only recognizes him in disguise. For the next nine years he will 'hold him close' as protection from Tywin, not realizing that Jaime is the enemy to be truly feared.
10. When Rhaegar goes to find him, if he does find him, Jaime tells him the story of Lyanna beating off the squires-in true knightly honourable fashion-and Rhaegar gives her the crown in recognition, thereby beginning the events of Roberts Rebellion.
11. One other parallel here: Jamie's most honourable action seems to have triggered the events and subsequent war of 16 years ago, just as his most dishonourable, pushing Bran from the window, triggers the war with the Starks and begins the entire series of ASOIAF.
Edited by Lady Barbrey, 12 July 2014 - 02:19 PM.