Hortensia is the older girl that Matilda and Lavender meet during one of their first recess periods at Crunchem Hall. She's about twice their age, already ten, and a veteran of battles with the Trunchbull.
She's the one who explains to them (and to us) who the Trunchbull is and what this monster's like. She's the one who tells us what kinds of things the students do to the Trunchbull, and what the Trunchbull does in retaliation. We hear from Hortensia what The Chokey is; we never actually see it for ourselves. So this girl is our background info guru. She tells us all about Crunchem in general and the Trunchbull in particular.
After Hortensia tells the younger girls a little bit about her experiences at Crunchem, they practically start hero-worshipping her. She's battled with the biggest evil they've heard of, and made it out alive:
Both Matilda and Lavender were enthralled. It was quite clear to them that they were at this moment standing in the presence of a master. Here was somebody who had brought the art of skullduggery to the highest point of perfection, somebody, moreover, who was willing to risk life and limb in pursuit of her calling. They gazed in wonder at this goddess, and suddenly even the boil on her nose was no longer a blemish but a badge of courage. (10.30)
Hortensia's actually a lot like Matilda (without being another prodigy, that is). She uses her brain and wits to figure out pranks to play on the authority figures that mess with her. Unlike Matilda, though, she's not so good at getting away with it. She makes mistakes and gets caught, which is handy for Matilda, who learns from Hortensia's missteps.
At the end of the day though, Hortensia doesn't care one bit whether she gets caught or not. She's too cool to worry about that because, frankly, she's got swagger. She doesn't just walk, she "saunter[s]" (10.67). She's also got a sense of humor; after a long and fancy speech comparing the Trunchbull to Satan, she lightens things up by saying, "It's a tough life. We all try to support each other" (10.35). Strange words from a ten-year-old, but with a nemesis like the Trunchbull, they're all too believable.