From what we've seen in the show she doesn't really talk much, preferring to "Mh-hmm." However this very much causes us writers a problem, especially if we want her to be more talkative.

So aside from using body language, how else can we express Marble Pie's feelings?

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Facial expressions and movements is the way to go. Even though she doesn't say much, you can convey her feelings by putting down her actions. Such as frowns, scratches of the arm, lowering her head, etc.