Unless you count her having been mind-controlled for a long time as leaving some kind of imprint, Terra's about as purehearted as you can get without being one of those Princesses of Heart. That can probably go for Mjrn, as well. Rydia is largely an innocent, but has some strong emotions, not always of the positive variety. She's largely come to terms with what happened to her, but could be set off by other things.