The Percys are like the Malfoys: old-money snobs who think that "blood" is the only thing that matters. Despite their conflicts, Garp shares many formative memories with their judgmental clan.
He loses his virginity to Cushie, after all. It's explicitly stated that their secretive hookup—a "final reward in harrowing times" (4.197)—sets the stage for his later sexual relationships, many of which are secretive hookups. Garp has so many emotions tied to his brief fling with Cushie that he's absolutely devastated by her death years later.
Garp has enemies within the Percy family, too, Fat Stew and Bonkers high among them. Fat Stew hates Garp from the start, attributing his lack of discernable lineage as evidence of being a "Jap" (2.146). Hey there, racism. As for Bonkers, well, let's just say that things get pretty ugly between them—ears are eaten and hearts are broken.
But Garp doesn't realize that neither Fat Stew nor Bonkers is his biggest enemy until it's too late. This role gets filled Pooh Percy, the youngest daughter who never seems to grow up, and Garp's assassin. She becomes the prototypical Ellen Jamesian: an outcast from a male-dominated society. While no one could agree with the actions that she takes, it's easy to sympathize with her struggles. Though Garp doesn't get a chance to do so himself, we think that by the end of the book, he totally would, too. For more on this, hop over to his page in this section.