Miss Rita L. Desjardin is a very important character in Carrie.
History
As the story begins, Rita at first feels the same disgust everyone else feels for Carrie. She even has a tendency to approve the other girls’ behavior during the shower incident for her hysterical reaction at her first period. However, when she realizes that Carrie didn't even know what a period was, she takes her side. She teaches her about it. She also wants to punish the girls that taunted her by having them suspended for three days and banned from the senior prom, but the vice-principal settles on a lesser punishment: a week of boot-camp-style detention in the gym; refusing to attend detention will result in suspension from school and banishment from the prom. In Miss Desjardin's view, the only reason the administrators didn't go along with her proposed punishment, because they are all ruled by men, and thus didn't really understand just how nasty the girl's behavior had been. Lead bully Chris Hargensen who was punished worst during the announcement of Miss Desjardin's punishment, skips the detention and thus is banned from the prom.The principal reprimands Miss Desjardin for cursing at Chris, but stands by her when Chris's lawyer dad threatens to sue unless Chris is allowed to attend the prom and Mis Desjardin is fired. At the prom, she talks with Carrie about her own prom night she was several inches taller than her date and felt awkward, but remembers it as a beautiful event and later, congratulates her for being voted Prom Queen. Chris had rigged a cord connected to two buckets above Carrie and Tommy and pulls it, when they ascend the stage, drenching them with pig's blood. Everyone laughed, including Miss Desjardin and later she wants to "console" Carrie, who pushes her aside with her telekinesis and slams her against he wall, because she realized, she was laughing and was still laughing inside. She received a bloody nose because of it. Once outside, Carrie uses her "gift" to wreak the school with the intention of killing everyone, including her and then leaves for home, destroying everything she passes. Miss Desjardin is one of the few, who survived by running to the fire door with the other survivors.
Miss Desjardin is one of the few survivors of the "The Black Prom". Two weeks after the catastrophe, she retires from teaching, saying she is consumed with guilt over Carrie and also, secretly, for having behaved like she did at the Prom and even makes the statement, that she would rather commit suicide than teach again because of what happened.
Personality
Miss Desjardin has a far darker character than in the films. In the novel she behaves in many ways like the other girls to the extent of confusing lack of social skills and strange behaviour with antisocial behaviour. Only after the shower incident she begins to realize, that she is behaving wrongly and begins to do something against it by punishing the girls. However, she still did it in the end to clean her conscience because of having hit her and there were therefore still traces of her former behaviour left in her during the Prom. That's why she laughed at Carrie after being the victim of the assault ignoring Tommy on the way, too, and then tried to "console" her, a consolation or "helping" where there still was this evil attitude to humiliate her, the attitude that she somehow deserved it, because she was strange, an attitude which lead her to hit Carrie in the first place.
Having become fully aware of what she had done after the disaster, in which Carrie intended to kill her, too, she resigns from teaching. She knows, she is guilty regarding her behaviour, knows that she was the one, who behaved antisocially, and knows, how much she has completely failed that day as a guardian and teacher and that this failure contributed to the disaster, in which she nearly died, too. That's why she makes the statement, that she would rather commit suicide than teach again and implies, how consumed she is regarding this guilt and shame, when she says "if only, if only".