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: