We hear about Marjorie before we ever meet her. Here's how Willbury describes her:
"Marjorie was my clerk. Very bright woman. She used to deal with patent claims mostly. Invariably she could understand most inventions better than their creators, so when I retired, she decided to go into inventing rather than sticking with law." (14.10)
When we first see Marjorie, though, she's "a very unhappy-looking woman […] hunched in a deck chair, reading a book of mathematical tables" (14.53). Why so unhappy? One of the workers at the Patent Hall has made off with Marjorie's new invention, and since he seems to have vanished, she can't prove anything without finding him first. She perks right up when Arthur shares pies from Mr. Whitworth with her, though, so Marjorie isn't one of those heads-in-the-cloud types who forgets that she exists on the physical plane, too.
She is really nuts about machines, though. When she first figures out that the Laundry has a beam engine, she sounds all excited, and "Her eyes were shining" (16.14). Yep, she's a real engineering nut. Heck, even her resizing machine (which causes loads of trouble) came about because she was curious. As she tells Willbury:
"I was very interested in the scientific principles involved in making it. I just wanted to see if it would work. I hadn't really worked out what it was going to be used for." (31.53)
Way to not anticipate the consequences, Marjorie. In her defense, though, she might have been hoping people would just use the re-sizer to shrink pies, thereby enabling them to eat tons of pie. That's what we'd use it for, anyway.
Marjorie takes inventing very seriously, and she confides in Willbury that her stolen machine "'is the culmination of the last two years' work. But in the wrong hands, it could be very dangerous […] and now it has either been stolen or lost!'" (14.66). Given her hard work, it makes sense that she'd be pretty upset right about now; that she's concerned about the harm her invention can do in the wrong hands, though, also shows that Marjorie isn't in the invention game for only her own benefit.
Further, when Willbury tells her that investigating the Cheese Hall might be dangerous, and maybe she should stay back, she replies:
"I'd like to come. I might be able to be useful and I—well, I'd like to help if I can." (16.117)
Yup—Marjorie's got brains, and she sure would like to share them, along with any brawn she may possess, too. When she finally spills the beans to Willbury—saying: "'I have a dreadful feeling that this is all my fault!'" (31.26)—we realize just how much Marjorie's been motivated by regard for the well-being of others. She explains about her resizing machine, and how Snatcher and his goons seem to be using it to resize underlings.
And when Marjorie then asks the million-dollar question—"'Where is all the size going?'" (31.46)—though no one knows just yet, she opens up a key line of questioning to help get to the bottom of what's going on in Ratbridge.
When they're captured by Snatcher's men, Marjorie gets shrunk, which is a bummer and also kind of ironic since it means she—the person who invented the resizing machine—is now running around in miniature herself.
In the end, though, Marjorie helps save the day by helping crank up the magnet in the Cheese Hall to reel in Framley, Snatcher, and his armed goons. There's a question as to whether the magnet is powerful enough to do all that, but Marjorie's answer is: "'It will be by the time I'm finished with it!'" (49.38). You go, girl.
Marjorie remains tiny until the fashionable ladies of Ratbridge hear of her resizing machine and demand to have their strangely shaped buttocks resized to normal. When this happens, Marjorie can resize all the mini-critters back to normal, and then return herself to normal as well. She's bummed when Willbury insists that her resizing machine be smashed afterward—she did invent it, after all—but she agrees that doing so is for the best.
As the book wraps, Marjorie moves on to other projects, such as a sustainable fuel source, with Arthur as her assistant. It seems like she'll get some great work done in the future.