When Rui interrupts Natsuo’s kiss, she silently judges him as she roughly drags her sister to her bedroom. In retrospect, kissing an unconscious Hina definitely wasn’t his finest moment, even if it was a kiss meant to put a lid on his crush on her now that they’re step-siblings.
The next day at school, a late, bed-headed Natsuo learns that Rui has transferred to his school, and she’s immediately the talk of the school due to her being cute. Natsuo isn’t clear enough with his words, and gets Rui to think he wants them to act like strangers.
But the mere fact they went off into the courtyard together to have a private chat is suspicious enough to Natsuo’s classmates, who are well aware both he and Rui snuck out of the mixer together. When he meets with Hina, she asks if she kissed him while she was drunk last night, and apologizes in advance if she’s too “relaxed” at home again.
Rui was surrounded by girls in the morning, but by afternoon she’s all alone; Natsuo knows something’s up and has a pretty good idea: Rui is socially awkward, standoffish and fairly tactless. It’s as difficult for her to make and keep friends as it is easy for her big sister.
Natsuo offers to help her by pretending to be a (girl) classmate, and Rui is still her usual self-defeatingly honest self, but the two end up engaged in a dialogue that soon captures the attention of the entire class, who think some kind of skit is going on. Suddenly, Rui doesn’t seem so hard to approach, now that they know who they’re dealing with.
That night, a nude Natsuo predictably walks in on Rui in the bath, but thank goodness for once it doesn’t result in a blood-curdling scream and/or punch. As Rui quite logically points out, there’s nothing he or she haven’t seen before (interesting considering she once said to “forget” their tryst ever happened).
Rather than send him out, Rui calmly invites him in, and it’s Natsuo who hesitates. First of all, she wants to thank him for giving her a helping hand in breaking the ice with her class. But she’s also curious about why he tried to kiss Hina.
At first she dismissed him as an indiscriminate womanizer, but spending the day at school changed her mind about that. So instead, she wonders if he was merely trying to cheer her up in light of her recent boyfriend troubles.
Obviously, Natsuo doesn’t know about Hina’s boyfriend troubles. After the three step-siblings have dinner alone together when their newlywed folks go out for dinner (with Rui emerging as the best cook of the sisters by far), Natsuo brings up the subject, and Hina just as quickly deflects.
In a spot of bad timing, her boyfriend Shuu calls and she goes out to the front of the house to talk to him, insisting Natsuo not go outside with her. Nearly a half-hour later she rushes in, shaking and clearly upset, but still refuses to admit to Natsuo that she’s hurting.
Then there’s a knock at the door and the doorbell rings repeatedly, and both Hina and Natsuo assume its Shuu. Rui ends up the one to get the door, and it’s just their folks, a little tipsy from celebrating their marriage becoming official.
In his first days as stepbrother to both Rui and Hina, Natsuo is already trying to help them with their troubles. Rui will probably be fine with making friends in class, but Hina’s problems will be tougher to tackle, especially since she’s so reluctant to be helped (and she’s well within her rights, as an adult, to not want to seek help from a kid).
Regardless of whether he can help Hina, the fact is Natsuo’s new family and living situation is not nearly as crazy as the initial premise indicated. Whatever he may have done with Rui in the past, and however he feels about Hina, a new and powerful element has been introduced to his relationships with both: he’s unconditionally there for them, and doubtless they’re there for him.