My understanding is that the heroine of the game started off immature and kind of a brat, but over the course of the game various events happened that basically forced her to grow up and grow out of it. The problem now is that Eumilia's level 99 presence has thrown things off so much that some of those events just aren't happening, and others will be actively stolen by Eumilia (basically she thinks "I might as well take care of this ahead of time" while not at all thinking of how important those events are for the heroine's groth). As a result, the heroine never grows out of being an immature brat.