As far as you are concerned, she is silly and simple-minded. Shallow, even.
Intelligence doesn't necessarily equate with emotional maturity. You know this better than most.
She is not mature.
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Instead she is prone and quick to anger. The reasons behind her outburts are inconsequential; they have happened without rhyme or reason, like dry lightning.
( she will set the world ablaze )
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When she is happy, she laughs. Guffaws, mostly. Throws her head back and shakes with mirth and the noise clanks out of her in discordant harmony. Like someone banging on a keyboard and producing a tune on accident.
You'd like to trace the line of her throat in characoal, sketch her out with thready lines that don't quite connect.
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But only that.