warning: This section is filled with spoilers revealing important twists of the plot. Read at your own risk only if you're up to date with the japanese release of the manga!
the relationship: At first, Hachi and Nobu share a regular friendship. But one day they realize they have much in common in their views of destiny and love. Hachi's a girl that has suffered much in all her love experiences, and after that conversation she realizes that Nobuo is a someone who would be truly able to mend her heart and with whom she could "be happy everyday". Nobuo, for the first time too, sees a little bit more of Hachi's personality: before that he treated her with the kindness he shows to everyone (well, he once thought she was cute but that was all XD).
After that, those feelings grow in both of them, but at the same time Hachi is involved in a very complicated relationship which is, again, making her suffer more than any other thing. Anyway, they end up confessing their mutual feelings and Hachi decides to straight things up with the other boy. They share a wonderful and tender relationship, but only for a short time. Hachi discovers she's pregnant (and the father is Takumi, that "complicated" relationship I mentioned before). She's confused, depressed and unable to open herself to Nobuo and let him help her. He also feels devastated and guilty for not having what it takes to help the woman he loves. Takumi is willing to marry Hachi and take care of the child, and a heart-broken and guilty Hachi accepts. So that's obviously the end of the relationship, romantically speaking ^^u