Presea seems like a quiet and lifeless child at first. But that is because of the Exsphere that has been embedded in her. It causes her to show no emotion and to not age. She is significantly older than she initially appears. And even after she is able to show emotion, she is still not used to the sensation and finds it difficult to express herself.
Presea does not appear in the story for a while

Here is a reference scene. Since Presea doesn't appear until late in the game, it's nearly impossible to find a scene that doesn't have spoilers. But I think this one will do.
https://www.youtube.com/watch?v=rOPW2RLrF1I