I have no respect for these people. Hina moves out because she, an adult, likes her new 17 year old stepbrother. She knows her sister Rui is into him. So what does she do? She tells Rui to basically be free and live her life (presumably with Natsuo), but then turns around and gives him a key to her new apartment so they can hook up. First off, she's a pedo. Second, she's a terrible sister. Third, she's disgusting and selfish (if you need more examples just look to her affair with that married douche).
My opinion of Natsuo isn't good either. He's got about four girls chasing after him and has a crush on his teacher / stepsister (Hina). So what does he do? He hooks up / makes them think he'll hook up with each of them. He's SO passive and feints innocence every time. He knows what he's doing, he's just hoping he won't have to face any consequences.
Rui is best girl, but she's too good for all of them.