Rinna

The protagonist's best friend's girlfriend.
She appears to be a cool, beautiful girl who is admired by those around her, but inside she is a bit of a naive girl.
She's particularly clueless when it comes to anything sexual.
Because of this, she has complex about her huge breasts, and begins to worry that the reason her boyfriend won't have a deeper relationship with her is because he's put off by the size of her breasts, so she turns to the protagonist for advice.