In the manga, Usagi was first entranced by Rei's beauty and followed her about onto the bus and then off at the Temple. After a harrowing death fight with Jadeite (his death, to be exact), Rei learns of her destiny. Afterwards, while Rei no longer entrances her, Usagi still admires this dark beauty very much for both Rei's wisdom and her calm boldness. But in the manga, Usagi does not have as good a relationship with her as some of the others. There seems to be more going on between Rei and Minako or Minako or Usagi, and didn't quite concentrate on Usagi and Rei. What a shame, really.
In the anime, Rei sensed some evil at her temple the moment her crows attacked a visitor, and thought it was coming from the blond girl that had followed her. So she threw her anti-evil scroll at Usagi and kicked her, knocking her unconscious.
Thus was a start of a beautiful, if not incredibly, rocky relationship. You can tell these two love each other. I mean, they fight like sisters! (I am an expert on this subject! I have three of them! . . . Wait, didn't I already mention this?) Rei is constantly trying to straighten Usagi out, correct her manners and habits, for she feels that Usagi cannot be the princess or leader if she does not mature. However, I think there is more to it than just this. Rei generally wants what is best for Usagi and she feels that Usagi, with her blundering sweetness and innocent ways may be refined into something far more richer, such as what Princess Selenity was and Neo-Queen Serenity is. Still and all, Rei tends to be somewhat bossy and pushy as she goes about this self-appointed duty in a somewhat rough manner.
There are several beautiful parts in the anime that prove this sisterly relationship. In the first half of the Sailor Stars season, when Nephernia had kidnapped Mamoru, Usagi turned into Eternal Sailormoon without the use of the Holy Grail, and flew off to rescue her beloved. Rei was the only one who tried to stop Usagi.
The second scene that I absolutely love was episode 197, when Galaxia took the starseeds of all the inner senshi as they protected the others. It was to Rei's side that Usagi ran to, calling out her name and hugging her close. Rei died with her hand in Usagi's hair, telling her that she would never be a good senshi, because she was much too loving to be strong. Indeed, this may be one of the reasons why Rei is so hard on Usagi, because she feels that Usagi needs to build up this strength and if people always indulge Usagi in her child-like matters than she would never learn otherwise.
And my absolute favorite (other than the reason it is for comic relief), was in the later half of the SailorStars season when the Metal Senshi had learned of Usagi's beautiful starseed and identity. The other senshi took precautions to protect her. This meant that Makoto and Minako didn't move an inch from her side--literally. In gym class, they did jumping jacks armpit to armpit with Usagi in the middle. But what I thought was sweet (and comical, while we are at it), was when Usagi was sifting through the bag of emergency things. "What's this?" she wondered as she held up a whistle. Makoto and Minako could not move fast enough to stop her as Usagi sucked in a deep breath and blew hard on it. Silence fell over the city. A cloud of dust rose up along the street and a rumbling was heard . . . And before them stood a huffing, puffing, and extremely worried Rei who was demanding to know what had happened.
All together, it was sweet of Rei to come as fast as she could when she did (during school). She would go through all ends to protect her friend, and did, as a matter of fact, time and time again, just as Usagi does for her.
I think the anime places a large emphasise upon Rei's and Usagi's relationship being more of a sisterly love than anything else. Some people may point out that there is lust on Rei's part involved in the matter because of Rei's passion, but I don't believe so. This may be proven by one of the episodes of the second arc of Nephernia's story in the Sailor Stars season. Rei found herself in a very unlikely team with Michiru on their way to rescue Usagi and Mamoru.
While running through the forest, looking for others, Michiru made the comment of Rei and Usagi being very good friends--like her and Haruka. At this comparison, Rei's jaw dropped wider than Usagi's. Yes, this screencap is her actual reaction. I believe what Michiru was trying to imply was that Rei's and Usagi's relationship could hold up under any circumstances and that they would go stop at nothing to help one another for they truly and absolutely loved one another, just as she (Michiru) did with Haruka. However, Rei made the assumption (following along my beliefs) that Michiru was comparing Rei's and Usagi's relationship with Michiru's and Haruka's along the line of being lovers. This is backed up by the smile Michiru gave Rei, which I felt meant that Michiru knew that Rei mistook her and she thought it was silly.
In the American version, many people think that Rei is a very nasty person who should be run over by the Amtrak train. I never thought that of her. On the episode where the others learned that Sailormoon was the princess, she began to cry, saying that she did not want the responsibility at all. DiC cut out the part where Mars slapped some sense into Moon. But in the DiC version, Sailormars says, "I know that I haven't been a very good friend and have wanted to be the leader because you are such a baby, but you need to be strong and pull together, for us and for Darien (Mamoru)."
Or something along that line. I can't be too far off. Why would someone who wasn't a friend say that? What I have been trying to say is that this relationship is so strong, that I can sense strains of it in the very chopped- up American version, and I couldn't sense a skunk unless it sat right under my nose. However, I cannot say anything for the rest of the people out there.
This relationship is one of the most important for Usagi, but I'm not going to really go into it because it's been beaten into the ground so many times by Rei-shriners to prove that Rei is not anti-Usagi (I was just tryinig to tell it from Usagii's point of view, but I don't think I managed to succeed). But if you really want to get into the detail, I would suggest this.