Wait, so she's basically just as bright as her sister was presented in chapter one. This character isn't a natural gloomy introvert, so basically it was the parents different treatment of the two that made her gloomy and the other bright.
Since we now see that she easily responds to love with love, she must have never received the same level of love and care as her sister, which would be odd unless we view the family as poor. They probably hated the fact they had twins immediately and decided to only treat one with proper care in order to save money. Remember how our mc was wearing tags and unkempt while the sister was well dressed and groom. There was probably a difference in meals as well.
Just what I've inferred so far.