Symbolism, Imagery, Allegory
We know, we know—it's pretty weird to call two of the main-ish characters symbols. We're actually even calling them a collective symbol, which doesn't help our case. Stick around, though, and we'll tell you why.
What's one of the biggest issues that plagues the book from start to finish? Cathy's alleged rape by Goodman. It's a constant source of ambiguity, and the characters obsess over it for several decades. In other words, collectively, Goodman and Cathy represent the huge, glaring failures of this group of friends Jules thinks is so perfect, and the slight shift in adulthood toward recognizing those failures.
When Goodman is arrested, his family (especially Ash) and Jules fail him by idealizing him instead of holding him accountable to the evidence. They let him run off to Iceland and keep going to see him and support him, effectively giving him a free pass (and showing absolutely no regard for Cathy, or other women who might cross Goodman's path). This doesn't help Goodman at all, though, and instead, he ends up a gross, lurking kind of guy who keeps trying to pretend he's the same suave, untouchable teenager.
When everyone takes up Goodman's side, they just let Cathy drop and quickly cut her out of the group. We don't really know what Cathy is up to in between the arrest and the brief moments she's on television after the 9/11 attacks, though we do know that the only character who seems to feel any remorse over how the group handles her rape claim is Ethan, making it clear that he's a better guy than the rest of the Interestings, if only slightly.
Cathy and Goodman are a constant reminder that this group of friends is so interested in its own, er, interestingness that they fail to address a real problem. And in doing so, they fail to recognize a whole lot of ugliness in both themselves and each other.