At twenty, Darry is the old man in The Outsiders. He's the oldest of the Curtis brothers and, even though he's over the hill by Greaser standards, is movie star gorgeous. And, like his brothers, he's super smart and athletic. (Plus, he does all the cooking and other housework.)
He's strong, principled, very serious, often grumpy, and can get violently angry. Pony tells us that before their parents died, Darry "had been real popular in high school; he was captain of the football team and he had been voted Boy of the Year" (1.81).
Because he's so physically fit, the gang calls him "Superman" and "Muscles" and Darry doesn't get upset about it (7.45). But, Pony tells us,
one time Steve made the mistake of referring to him as "all brawn and no brain" and Darry nearly shattered his jaw. […] Darry had never really gotten over not going to college. (7.45)
When the Curtis parents died, Darry took on the responsibility of raising his two teenage brothers. So, as Ponyboy constantly reminds us, Darry works all the time at two jobs, one of which is roofing. He doesn't have much of a life outside of work, the gym, and his responsibilities at home.
Ponyboy's feelings toward Darry are conflicted and complicated: they struggle to understand each other. Since we see the struggle through Pony's eyes, though, we can only guess about Darry's journey based on the details Pony provides.
We think he's so hard on Pony because a) he sees Pony's potential and is afraid that if he isn't hard on Pony, then Pony won't reach that potential; and b) he has no experience as a parent! Darry's stepped into this role without any preparation, and is probably really afraid to fail. His little brother's safety is all in his hands, and there's danger lurking around every corner. When Pony doesn't take care of himself or act responsibly, Darry feels frustrated and angry. He has pressure from all sides, and is always on the verge of exploding. According to Pony, Darry wasn't like this before their parents died.
We aren't trying to excuse Darry for hitting Pony that one time. Pony says "Darry wheeled around and slapped me so hard it knocked him against the wall" (3.103). For Pony, who had never before been "hit" (3.103), the act is unforgiveable, even though he hears Darry's apology. For all he knows, the violence that plagues him on the street has now seeped into his own home.
For Darry, it's the beginning of a lot of guilt. If Pony hadn't run off that night, Pony and Johnny wouldn't have walked to the park and, well, this would be a very different book. Plus, Darry basically loses Pony for an entire week. He must have been beating himself up like crazy.
It's only when Pony realizes that Darry loves him (but has a hard time taking on the responsibilities of being head of the family) that Darry begins to really soften. Pony comes to see that Darry is little more than a kid himself, and one who has chosen to care for his brothers and give them a good life, rather than lead the relatively free and easy life of most twenty-year-olds.