Rocky is about as straightforward of a story as you can get. We meet Rocky. We get a preview of his boxing abilities: he's sturdy, but sloppy. We follow him through the mean streets of Philly, and see how he's desperate to catch a break. Then the break comes, and we watch him train until the championship bout at the end of the movie.
Interspersed with the boxing story of Rocky is the love story of Rocky and Adrian… which follows the exact same path. Rocky is a sturdy, but sloppy man with a crush. He's desperate to catch a break with Adrian. The break comes, and, at the same moment he's finally "gone the distance" with Creed, Adrian confesses her love for him. D'aww.
Stallone is in almost every scene, except for a couple brief asides showing Apollo Creed planning the big fight. It's Rocky's story through and through: that's why his name's on the poster.