Sir Leye is one of the Temple Knights that test your skills during the Recruitment Drive quest. You must fight him as a woman, using no armour, weapons, food, prayer 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 life points reach zero, they will automatically reset to its maximum, making him invincible. However, if you try to kill him as a female character, his life points won't reset back, effectively killing him. After attempting to fight him about 4 times, you will no longer have to face him in the quest. You do not get any experience while fighting him.
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.