Le Morte D'Arthur
How we cite our quotes:
"What knyght ys that," seyde Sir Trystram, "with the blacke shylde and blacke horse?" "I know hym well," seyde Sir Persides. "He is one of the beste knyghtes of the worlde." "Than his ys Sir Launcelot," seyde Sir Trystramys. "Nay," seyde Sir Persides, "his is Sir Palomydes, that ys yett oncrystynde." (313.29-33)
An important part of Sir Palomydes' identity is his religion. As a Pagan, but one who nevertheless fights well, he presents a problem to the story's Christian values-system. But that problem is quickly solved when Palomydes finally receives baptism at the end of the Tale of Trystram. So now his devotion matches his skill on the battlefield, and Le Morte always prefers it that way.
"Why," seyde Sir Trystram, "woll ye do batayle wyth me but yf I telle you my name? Forsothe, that lytyll nedyth you? Abnd ye were a man of worshyp, ye wolde nat have ado with me, for ye have sene me this day have had grete travayle." (337.19-22)
The King is the most important "identity-giver" in Le Morte D'Arthur, since all of the people in the story define themselves through their relationship to him, whether as his vassal, wife, brother, sister, etc. So Trystram's decision to keep who he is a secret from Arthur might mean that he's not quite ready to participate in this identity-giving system just yet.
Than at the last Sir Launcelot spake and seyde, "Knyght, thou fyghtyst wondir well as evere I sawe knyghte. Therfore, and hit please you, tell me your name." "Sir," seyde Sir Trystram, "that is me loth to telle ony man my name." "Truly," seyde Sir Launcelot, "and I were requyred, I was never loth to tell my name." "Ye sey well," seyde Sir Trystram, "than I requyre you to tell me your name." "Fayre knyght, my name is Sir Launcelot du Lake." (343.43-49)
Why is this the moment when Trystram finally chooses to reveal his identity? Maybe it's because Launcelot is the knight he most respects. So now that he has battled him to a draw, he feels proud enough to admit his name. Plus, Launcelot has told him his name, so it's only fair.