His relationship with Darry is almost like one with a father. He feels more distant from him than he does from Soda. That's one reason why they fight so much -- Darry is trying to act like a father to Ponyboy and that is hard on both of them.
Ponyboy and Darry didn't get along with each other at first, due to the fact that Darry doesn't want Ponyboy to fail school like him. But soon Ponyboy found out why he was tough on him and got along with Darry. He was always good friends with his brother Soda and never got a problem.