When Arthur accidentally takes refuge in a closed shop, he meets Willbury, who is an older-looking dude " wearing half-glasses and a gray wig and […] smiling at Arthur" (5.9). Seems friendly enough, right? And then he inquires about who Arthur is in "a gentle voice" (5.10) and immediately offers his assistance. In other words, go ahead and put this guy in the good guy camp.
Willbury is downright honorable when it comes to helping others, so when Arthur first tumbles into his shop, and Willbury gets the chance to talk to Grandfather on the doll-phone, he says:
"I would just like to say that you have my word, as a gentleman, that while your grandson is in my charge, I shall do all within my power to keep him safe." (5.80)
Not only is Willbury keenly aware of the right thing to do, he's dedicated to doing it. Which is lucky for Arthur since he's just a kid above ground with his Grandfather no where in sight.
When Willbury finally gets to meet Grandfather, he says that he and his group will do anything to help Arthur out:
"Arthur has become very dear to us and we all want to get him back as soon as we can." (40.14)
Which leads us into…
For all the stereotypes and jokes about lawyers out there, Willbury (who's a lawyer) is actually very caring when it comes to people he knows. For instance, when his home is broken into and his monster friends are kidnapped, he freaks out, asking what's happened in a whisper, "The color […] gone from his face completely" (15.2). His voice is "shaky" (15.4) and he asks Arthur urgently, "'Where are the creatures?'" (15.4).
He is relieved to see that the mini-critters are still okay, and he "mournfully" (15.24) tries to comfort them. He suggests that Marjorie look after them on the ship (16.116), clearly showing that he's concerned about their welfare.
This concern for others means that Willbury will totally chew someone out if they don't do what they're supposed to. So, for example, when Tom and Kipper let Arthur sneak into the Cheese Hall alone, Willbury harshly reminds them: "'You're supposed to be looking after him!'" (24.5). Willbury: A dude who knows what's right, and expects others to also.
He manages to be really diplomatic, though, too. When Herbert's iron socks come off for the first time in years, Willbury kindly says, "'I think you had better go and wash those in the canal'" (47.19), which is a very polite way to let Herbert know his feet stink. This shows regard for Herbert's feelings, in addition to an ability to act in tricky—or stinky, as the case may be—situations.
As Arthur gets to know Willbury, he finds out that he was once a lawyer, but now he's retired. It sounds like he had a pretty active career, though, and he reminisces about taking down failed patent acquisition officers, saying: "'When I was a lawyer, it was a favorite pastime of mine to break the grasp of that filth. I have very happily kicked their posteriors on a number of occasions!'" (14.50). He even goes so far as to throw a turnip at them (14.48).
Apart from vegetable-flinging, though, Willbury tends to stick to the rules. He tells Arthur, "'I don't hold with taking other people's things'" (7.25), and he gives the boxtrolls grief for "liberating" (read: stealing) mechanical parts all over Ratbridge.
It's not a bad thing that he's so principled, though. When Gristle comes around trying to buy the underlings from Willbury, he responds by saying:
"I'm warning you. GO AWAY! I do not sell friends!" (11.29)
We hope you only ever have friends who agree with this sentiment. And if not, may we gently suggest you spend some time thinking on that.
At the story's end, he lays it thick on the corrupt Chief Squeaker, saying that he plans to return to practicing law, and that the Chief Squeaker should watch his back since he was obviously collaborating with Snatcher. In fact, Willbury goes on to clear Grandfather's name and secure a new place for Grandfather, Herbert, and Arthur to live aboveground, and even helps pay their rent until the lawsuit against Snatcher comes through with some money. Having Willbury on your side sure is handy.