Rei is one of the more mysterious ones in
the Inner or Outer Senshi. She has had an unusual and
reserved upbringing, living at the shrine with her
grandfather. She also has not had an easy time with her
looks and her background making her a bit distant from her
schoolmates, prior to meeting the Inners.
While Rei does not show the lack of
social skills like Ami, she is not very good at showing her
actual feelings to the people she cares about. Usagi is the
best example of this, with Rei truly caring for her so much,
yet being unwilling or unable to let that love she has for
her friend show.
Her attitude towards Yuuichiro confirms
that Usagi isn't the only one that this behavior is limited
to. She does care for him as well, but does not let it show,
either. We must assume that he has come to realize that
fact, or he probably would have long since moved on in terms
of seeing someone else.
All of the others seem to have mostly
caught on to this behavior of hers, actually, with Usagi
being the only exception. Usagi probably is only beginning
to get the slightest hint of what Rei is truly feeling
behind the facade she puts forth. Rei seems to be trying to
make Usagi into the more ladylike person that Rei feels she
should be since she is the Princess.
Also of note is that Rei, as a miko, or
shrine maiden, has some knowledge and understanding of
people that a girl of her age would usually not have. I'm
sure that part of her training has included learning about
people from her grandfather, and getting advice and
instruction from him is going to make her more perceptive
about people than most of the other Inner Senshi. When you
combine this with her apparent precognitive abilities, she
will probably be the one to first get an idea of what a
person is like and what their real motives are.
In regards to the Outer Senshi, I do not
see any one of them being more of a choice as a friend over
the others. I think Rei would get along well with all of
them equally well, and be able to deal with any of them on
an individual or group basis. Rei seems to understand the
moral basis for the decisions the Outers have made.
Undoubtedly, Rei's time with her grandfather and learning
from him about people and what makes them feel the way they
do, since she is the probable successor to the shrine, will
make her more understanding and accepting. While Rei may not
agree with what the Outers feel and the decisions they have
made, I do see her being the Inner Senshi who can most
easily understand them.
Rei's most notable characteristic is her
dedication to Usagi. It has been said that she is probably
Usagi's best friend, and I would readily agree with that.
Though she constantly fights with and criticizes Usagi, she
genuinely cares for the princess. Mars is always the one who
protects Sailormoon, as in episode 45 when she kills two of
the youma to save her and in the R movie when she pushes
Moon away from Fioret's flower monsters. And it's clear that
behind the teasing, pushy exterior that she is fond of the
blond, she is just not good at showing it.
The two main male relationships in her
life are Mamoru and Yuuichirou. I tend to sincerely downplay
her feelings for Mamoru; quite a few fics I've read try to
make it into the biggest deal that has ever happened in
Rei's life when it's not. Rei-chan, in my opinion, felt a
strong infatuation for Mamoru-san, that was it. Of course
she was hurt when she found out about how Usagi and he were
meant to be together, but she manages to push away that hurt
for her friend's well being. In any case, her relationship
with Yuuichirou develops past just friendship, though Rei
doesn't like to acknowledge how much she cares about him
Rei has many possibilities open for her
in a fanfic. I would not suggest emphasizing her desire for
Mamoru long after their relationship has ended [ie like six
years after the first season] because that is really
ridiculous. Her strength of character, determination,
attachment to her friends, and her quick temper can be used
well, if the writer knows what s/he's doing.