I like to think that the new character, Sana, feels burdened of the high ideals of being an imouto or young lady in the school. Like shackles in society, until one day, someone so bright and free, the gayru Ririka shows her the joy of being carefree, the joy of freedom. Thus, she tries to weave her fate by her own ideals and not the ideals of others. Which starts with trying to go out with the gayru.
Overdramatized? Definitely, doesn't fit this story's themes at all. But its fun having your brain suddenly over fantasize the moment you see something.