Hester is ugly. Oogly, even. She fell out of the ugly tree and hit every branch on the way down. And that sucker was a Redwood, the tallest tree in the world. This isn't our opinion: Tom tells us on almost every page how ugly Hester is. Why is Tom so repulsed by Hester? Because of the giant scar on her face.
When Valentine hacked Hester's face in two, she lost an eye and was left with a scar that permanently twisted her entire face. It's a horrible reminder of the day Valentine killed Hester's parents—and almost killed her. She was only about eight years old. No wonder she "always expected the worst and was usually right" (8.29). It doesn't help that people can't shut up about how ugly she is.
Ugly might just be skin deep, but Hester's ugliness runs a little deeper. She is consumed by hatred for Valentine and driven by the desire for pure, hot revenge. That makes her kind of ugly on the inside, too. Remember when she lashes out at Tom for saving her life?: "Why do you keep interfering?" (22.34). Hey, he could have just let you die, Hes.
Hester's anger seems to cool a bit by the end of the novel, though. Maybe it's because she finally witnesses something good in her horrible life when Katherine sacrifices herself to save Hester. Well, that's good for Hester, anyway. Hester resigns herself to the fact that life may not be perfect, but it does go on, and you just have to live with it.
When Hester spills the beans about her tragic past to Tom, she says, "I didn't even cry when Valentine murdered my mum and dad" (4.18). We're not sure if that's because she's hardened her heart or because Valentine's sword severed her tear ducts. She probably wouldn't even cry after reading a Nicholas Sparks novel.
Hester's ruthlessness can come across as coldhearted, but numbing her feelings takes work. When she decides that becoming a soulless Stalker like Grike might be a good idea, we definitely get the sense that she's getting tired of maintaining her stoic facade and is looking for other outlets. She confesses that the transformation "would make things so much easier" (26.14). For Hester, it seems easier to remove your feelings than to confront them.
After the explosion of London, though, Hester does confront her feelings. And she cries. "She was crying, crying for her mum and dad, and Grike, and Katherine, and even for Valentine as the crackling light around the cathedral grew brighter" (36.11). She's pretty much crying because the world is broken and there is nothing she can do to change it. Her whole mission has been to kill Valentine, but that won't bring back her parents, and it won't fix anything. Man, being a Stalker is starting to sound like a good deal to us after all.