Fortunately for Ella, Olive is not as mean or manipulative as Hattie. Although, you get the sense that she would be, if she could. Unfortunately for Olive, she's just a little dim.
When Ella meets Olive for the first time at Lady Eleanor's funeral reception, Ella thinks: "Olive's face was as blank as a peeled potato" (3.52). And Olive's insides are as blank as her outsides; basically, Sir Peter nails it when he calls her a "simpleton" (4.125).
But Olive is smart enough to know there's something special about Ella. She looks up to her, like when she writes after Ella's great escape from finishing school: "Hattie says Ella was bad to leeve but I think she was bad not to tak me to" (16.34). And when Sir Peter and Dame Olga marry, Olive stands outside the hall with Ella and asks "Can I stay out here with you?" (20.29).
Whatever Olive's reason for looking up to Ella—perhaps she's never had a positive role model in her life before, or perhaps it's because Ella's not mean to her like Hattie is—it makes the Olive-Ella relationship a bit more complex than the wicked stepsister paradigm suggests.
However, Olive still manages to torture Ella when they become stepsisters. She demands that Ella pay attention to her, tell her stories, and stuff like that. Ella's only recourse is to escape to the library for moments of privacy, since no one seems to think to go there ever—especially not Olive, whose reading and writing skills leave something to be desired.
Olive is, to put it mildly, not very intelligent. She doesn't even figure out Ella's curse, only getting as far as the realization that if Hattie orders Ella to do something, Olive can then ask for the same thing.
Her basic math skills are also lacking, as shown when Olive says to Hattie after the first ball: "You said you'd give me three coins for every time he couldn't dance with anyone else because of me. You owe me … Eight coins" (27.56). Seeing as eight's not even divisible by three… yeah. Something's off there.
Thanks to the magic book, Ella gets to read one of Olive's letters home, full of terrible misspellings and grammar. (Good thing Ella's skilled with languages.) As an example, and we apologize if it gives you a headache: "I hav ben feeling poarly all week. I hav hedakes espeshly wen I reed" (16.33).
Poor kid. Maybe they don't diagnose learning disabilities in Kyrria, or maybe she just needs a new pair of glasses, but clearly no one's ever invested much time in teaching her how to read and write properly.
There's one thing that Olive loves: money. During the carriage ride to finishing school, after Ella's forced to give her mother's necklace to Hattie, Olive asks for a gift too: "I want money. Give me money" (8.10). When the stepfamily finds out that Sir Peter is poor, Olive panics and asks: "Is our money gone? Will we starve?" (22.21). Coming from a rich family, Olive has clearly never been in danger of starving, but she seems to have internalized her greedy mother and sister's view of things: more money is always better.
She also, however, seems starved for attention. Either Dame Olga has spent more time grooming Hattie to be the eligible-to-wed daughter (for all the good it's done), or Dame Olga's parenting skills are non-existent. (Or both.) Either way, Ella moves in with the stepfamily, and Olive hounds Ella with orders to talk to her, tell her stories, count her money and generally pay attention to her.
In the end, we know that Olive isn't really to blame for her personality, because she gets a happy ending that meets her need for human contact: she marries a "garrulous widower who fell in love with her unwavering attention" and is rich enough even for her. (Epilogue.9). Still, the fact that Olive seems to have spent so much of her young life neglected and indoctrinated with greedy values makes us kinda sad for her.