The relation between Orihime and Rukia is one of the sweetest things of the first arcs of BLEACH.
They start their relation as schoolmate, and with the development of the series they have the chance to become closer than they could think.
Orihime and Rukia are not just comrades or schoolmates, they're a sort of "rival in love", but in fact, they don't consider the other one like that.

Rukia always supported Orihime and when finally they have the chance to discuss about their respective feelings, Rukia smiles to her saying that we're friends.
I always thought that they could be not only a great team but wonderful friends, and Rukia dried Orihime's tears in difficult moments for her...
Orihime can't forget how Rukia is important for her, she'll never forget how Rukia's smile did for her sake. I really loved the moment in which Orihime left Kawakura Town in order to reach the Soul Society and save Rukia's life, but I loved more and more when Rukia left not only Karakura Town but her own world and its rules in orther to save her in Hueco Mundo.
Rukia is a shinigami with a great talent and an important position, but even if Yamamoto said to her to stay home, she left her home at once, thinking of Orihime's sake.
Truly, they're so adorable... more than friends buy not yet like sisters, because they have a lot of things to find about the other one.
I think that a friendship developed since the childhood is awesome (like the one etween Orihime and Tatsuki or Rukia and Renji), but the ones born in difficult moments with a person that you don't know it's sweeter than it.
How splendid is when you change your idea about a person that you thought diffident, finding in her an incredible friend.
Rukia and Orihime's song is called "Holy Fight" and I chose it like title or their fanlisting because that song is part of their hearts and their relation.

If you want to know something more about the song...

Download: MF (Thank you to Takuya for the link)