Though Buddy is telling the story of Seymour, he is centrally a part of the narrative. We learn as much about him as we do about his subject, and so we've elected to consider him a central, rather than peripheral, narrator. Feel free to argue with us.
Buddy is a present narrator. In other words, he's always reminding you that he's narrating. He's also a tricky narrator because he plays with the reader, the story, language and form. In "Roof Beam," he'll interrupt the narrative to comment, or to address the questions he's imagined you're asking (as in, why did Buddy get into the car with the guests? Because he was lonely). In "Seymour," Buddy spends more time digressing and commenting on his content than he does narrating the content. (Paragraph-long parentheticals in the middle of a sentence? Par for the course.)
Buddy's role as a narrator is an important aspect of his character. To talk about the narration, we really have to talk about Buddy altogether. Be sure to take a look as his "Character Analysis" to get a larger sense of who Buddy is.