Third-Person Omniscient Narrator
Scrap is the WBA cruiserweight champion of eavesdropping.
Whether he's taking his sweet time buffing the turnbuckle or hiding in a shadowy corner of Maggie's rehab center, he always seems to be in the right place at the right time. Is that rude? Maybe? Is that an awesome skill for a narrator to have? Definitely.
Scrap's an omniscient narrator. That means he's all-knowing and all-seeing. (Well, almost all-seeing. Sorry about your eye, Scrap.) He has a wizard-like ability to jump around in time, from event to event, and even give us insight into the other character's thoughts.
Unlike your textbook omniscient narrator, though, he's not inside the other characters' heads. Scrap doesn't divulge their private thoughts, at least not directly, and that's important. The conceit of the story is that Scrap's not really narrating the movie for us; he's telling the tale to Frankie's estranged daughter, Katy, in what must be one seriously long letter.
As a narrator, Scrap can still give us astute glimpses of the other characters' inner workings, though. Like when he tells us that Maggie "grew up knowing one thing: she was trash." It's totally plausible that Maggie told Scrap that directly, or that he read between the lines of one of her stories.
Similarly, while Scrap himself is part of the story, he also provides voiceover for events he didn't attend—like Maggie's knockout tour of Europe, for example. The thing is, all of these are events where it's reasonable to believe that Frankie or Maggie told Scrap what went down after it was over, and he's just relaying that info to Katy, and us.