Sir Leye is one of the Temple Knights that test your skills during the Recruitment Drivequest. You must fight him as a woman, using no armour, weapons, food, or potions. This is due to a famous legend stating that Sir Leye had been blessed by Saradomin himself, and that no "man" can defeat him. If your character is male, you can damage him as much as you want, but even if his Hitpoints reach zero, they will automatically reset to its maximum, making him invincible. However, if you try to kill him as a female character, his health won't reset back, effectively killing him. It is guaranteed you will have to fight Sir Leye during the quest, Recruitment Drive, as he is one of the seven tests that is chosen randomly every time you re-enter the testing grounds.
His invincibility against any man may be a reference to the prophecy of Glorfindel, featured in Tolkien's novel The Lord of the Rings, for the Witch-king of Angmar fell not by "the hand of man". The prophecy fulfilled as he is killed by Éowyn, princess of Rohan.