Although we don't actually meet her (the dream doesn't count), Ellen, Lyman's ex-wife, is crucial to understanding Angle of Repose.
First off, there's no disputing the basic facts: Ellen did wrong by Lyman. Seriously, folks, she pulled a trifecta of bad moves:
In other words: ouch. So, while Lyman goes a bit overboard with his Ellen hatred, he certainly has reason to be upset. Wouldn't you be ticked off, too, if you were in his shoes?
Nevertheless, Lyman could probably cut Ellen some slack. After all, she recently suffered a serious tragedy of her own: her new husband went missing one day, and "it wasn't until the following summer that some fisherman found his bones in a ravine" (7.7.58). While it certainly would be asking a lot of Lyman to be there for Ellen given the circumstances, we think it's a lot worse to imagine him abandoning the mother of his child in her worst moment.
The end of the novel shows Lyman realizing this fact. More importantly, it shows him realizing that he could be the one to build bridges between the two of them—after all, he was the one who tore them down in the first place. And, who knows: maybe Lyman could actually use the company, too.