I feel like, for me, the most interesting thing in this manga is Namami and Sora's relationship. They have been together ever since Sora was born, and technically get along really well, but at the same time they compete for the big bro. Yet, they can't ever compete for him equally. Sora will always be his one and only little sister, no matter what happens, good or bad, whereas Nanami will be nothing but a stranger, unrelated to him, if things turn sour. On the contrary, if things go perfectly Nanami's way, she and Riku could be bound together forever, with her being even closer to Riku than Sora. Both Nanami and Sora do know all this, which is why it's so interesting to observe how they treat and behave in each other's company.