Squire Asrol is found in the east end of the courtyard of White Knights' Castle. Speaking to him starts the quest The Knight's Sword, where he is in trouble and needs help from an adventurer. He also can be talked to in order to start the Falador Tasks and to obtain the rewards when completed the Falador Hard Tasks.
Also when you start the quest Wanted! he says you have to spend five years being a squire before joining the white knights.
Apparently he is still a squire even though having served more than five years as a squire.
In the Knight's Sword, he explains that he has lost his master, Sir Vyvin's, sword. Since it's a very special ceremonial sword, he would be in big trouble if Sir Vyvin found out.
The squire is also an anagram in a level three clue and will give you a puzzle box to complete for the clue.