Let's be real here: At the beginning of this book, we didn't like Pop very much. After all, he threatened his grandson with an ax and he's a raging racist. Plus, according to Jessaline, "Everyone in Katoomba knows him because he's such a loudmouth" (38.30). Good grief, right? Talk about Bad Grandpa. Still, one of the main themes of this book is that there are multiple sides to any family's story, so let's see if we can't figure out Pop's.
To begin with, Stan's one of those old dudes who thinks everything was better in the good old days, often to a fault. He's an ex-cop, a guy who's job was to enforce order, but check him out now:
Stan could never find words for the way the world kept changing on him these days. [He] would feel like some kid in a fairy tale, a kid who'd been asleep inside a mountain for a hundred years and then woken up in some foreign, unfamiliar land. (21.8)
This disconnect with the changing culture around him explains pretty much everything we don't like about Pop, from his criticism of Lonnie's lack of direction to his racist leanings. Everything, for Pop, feels strange and unfamiliar. And while he doesn't navigate this well, it also has to be really hard to go through life this way.
Fortunately, Stan ultimately realizes that he has the power to change, though it takes an unlikely person to cross his path in order to do it. Stan doesn't exactly meet Rose under auspicious circumstances—she comes after him accusing him of accusing her of eating cats because she's Chinese (yeah, it sounds weird and terrible to us, too). Still, because of this misunderstanding, Rose teaches Stan that despite their gap in age and different racial backgrounds, they have a lot in common—mainly, that Clara and Lonnie think they're "the enemy" (23.77). Good times.
Stan's friendship with Rose is a giant step toward him overcoming the stronghold of his rigid old-schoolness. While Lily and Lonnie are apprehensive about how Stan will react to Clara and Lonnie's engagement, they're unaware of how his brief encounter with Rose is changing his heart.
By the time Stan realizes that Rose's daughter Clara and Lonnie's Clara are the same person, he's accepted the idea. Seeing Stan meet Lonnie and Clara's train, kiss Clara on the cheek, and allow her to try on his mother's wedding dress paint a remarkable picture of his transformation. When it comes to Stan, you can totally teach an old dog new tricks.