The Young Boy is one of the two destined children. Orphaned at a young age, he was raised by the elephant people of Rashkuta, until the monsters sent by the Abyss Lords separated them. He escaped, living by himself the rest of his life, just waiting for his fate to catch up to him.
Young Boy can be found in any town, though he can't be recruited, just telling to be left alone whenever you try talking to him. Only Sarah is able to recruit him early. When you reach the final Abyss Gate, he will be there, and as you defeat the Abyss Lord Sarah will push him and enter the gate. If you have six characters at this point you will be forced to make room for him. You are forced to keep him for the rest of the game, though he will leave the party once you reach the final boss.