Areida is Ella's only friend at finishing school. Like Ella, she's a little bit of an outcast: she has a funny accent, and her family is poor. Ella isn't poor yet, but she knows all about not fitting in. So, the two bond over stuff like the Ayorthaian language (Areida's native tongue, which Ella is a whiz at picking up) and devising ways to get out of being proper. And when we take a look at Areida, it's no wonder that Ella becomes her friend.
Aside from Char and Mandy, no one has bothered to comfort Ella for the loss of her mom—not Dame Olga, not Hattie or Olive, and not even her dad. So it's no wonder that, when Areida sings a beautiful Ayorthaian mourning song to Ella, she cries.
And then, since Ella doesn't want to go inside with a red nose from crying, Areida agrees to watch her nose until it stops being red. The combination of nose-watching and nose-crinkling cracks up the girls, and they giggle an inappropriate amount (according to Manners Mistress, at least).
Areida is such a good friend that Ella's well and truly heartbroken when Hattie commands her not to be Areida's friend anymore. This seems like almost the worst thing that Hattie could do to her: "I'd thought of injuries, and I'd imagined terrible embarrassment, but I'd never thought of this kind of hurt" (12.2).
In this light, running away from finishing school is one of the best things Ella could've done, because it spares Areida from learning that Ella has been forbidden to be her friend until Ella manages to break the curse on herself. We figure Areida's strong enough to survive that since she weathers the mocking at finishing school, but still. Major bummer.
(Okay, technically Areida isn't a lady because she doesn't come from a noble family; this is part of the reason why the snobby girls at finishing school are mean to her. We just liked the alliteration.)
After Ella deliberately breaks Char's heart by letter, Areida writes a journal entry that shows up in Ella's magic book. Areida asks Char whether he knew what happened to Ella after she ran away from finishing school, and defends Ella when Char says she'd married for money.
We could all use friends like Areida.