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.