Sure, we never actually meet Brian's mom in the story, but she sure has an effect on our main squeeze, Brian. He's tormented by thoughts of "the Secret" he knows about her and by his memories of the happy family life that he had before his parents' divorce.
So what can we guess about her from the few clues that we get? Well, we know—from the wistfulness of Brian's thoughts of his pre-divorce family, and the specific memories of how she took care of him—that she loves Brian, and she's a pretty good mom. So that's nice. But we also know that she cheated on her husband (Brian's dad) and later demanded a divorce so she could continue to see the other man. Not so nice.
But you can't have it both ways, can you? She's either a good character or a bad character, right? Well, actually, our big take-away from Brian's mom may just be that she's both good and bad. Or, as they say, it's complicated.
That's what Brian is struggling so hard to come to terms with. He's just learned what Shmoop already knew: that moms can be mixed-up, messed-up, know-what's-right-but-do-what's-wrong people, just like everybody else. But you still have to love them—because they're moms.