At first galnce it seems as though these two hate eachother, but as the anime progresses you realize that they are actually best friends. When Rei first joins the Sailor Scouts she doesn't think that Serena should be leader. Rei makes it known that she thinks Serena is a whiney cry-baby who can't take care of herself. Rei soon changes her opinion though, and she does it subtly. Remeber the episode where Serena had given Rei the Cresent moon wand to protect in case something happened to her?(In the hideous DiC dub she left it under Rei's bed.) Why would she give it to Rei instead of one of the other scouts? The answer is easy. Because she trusts Rei the most,  and in the end of that episode Rei put all her trust in Serena and gave her the wand back. What about in the Sailor  MoonR movie when Fiore kidnaps Darien and Serena is inconsolable untill Rei says that everything will be all right and that they will get Darien back.  Another example of thier friendship would be to look at all the times the sailor scouts die(and are reborn). Serena is distraught when the others die, but when Rei is the one to go she always begs and pleads with Rei to stay and says that she can't go on with out he. Rei is also the only one that Serena stopped everything that she was doing and held her until she was dead. Now you tell me, if you hated someone would you do all that? No. I didn't think so.
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