Sailormars, in this time and place, is a mysterious girl named Hino Rei. Rei is a full priestess at Hikawa Shrine, the local Shinto temple. She goes to a first-rate private school (T-A Girls School), which also sets her a little apart from the other girls. She specializes in etiquette and the military arts; she is good at archery. She is a rather mysterious person, deeply connected to the spiritual world and able to sense evil if it nears. She is the mystical one of the group, the one most attuned to nature and most sensitive to otherworldly happenings. She is mature, serious, composed, determined... and does not speak much about herself.
As part of her priestess abilities, she has a strong link to the spirit world. She is able to read the sacred fire to find evil, and visions also come to her in dreams. She is the only one of the senshi who has otherworldly abilities even when not in senshi form - she has a slip of paper that she throws at things, shouting, "Evil spirit, begone!" This almost always banishes whatever evil she was fighting. (The first time she met Usagi, she stunned her with this technique.) Rei has long purple/black/red hair, (I assume that the differences I've seen are due to artistic lease, and I'm certainly not going to argue with Takeuchi-san) and deep purple eyes.
Rei likes white Casablanca lilies, and she can play the piano. Her mother died when she was very small, and although her father was a wealthy politician, he was also too busy to raise his daughter. Consequently, she was sent to her grandfather at the shrine on Sendai Hill. She is determined not to marry and make her children unhappy. Because her past contains loneliness, her principle is not to brood on it. Men are captivated by her quiet beauty, but she does not return their interest. According to Rei, she doesn't need love - she has, instead, kindred spirits with the same goal as she does: to defeat the enemies of Princess Serenity. She has a close friendship with Minako, half-competitive, half-respectful.
Rei tends to be stubborn and a little prissy at times - she won't hesitate to chide others for being the slightest bit disrespectful. Her priorities are set in stone; her gut reaction to being a Sailor senshi was along the lines of "Why must I be a soldier of justice? I'm very busy." Rei tends to act as if she's a cut above everyone else, and as if she's always got something better to do. But I think (if you'll allow me some amateur psychoanalysis) that Rei's seeming arrogance is just her way of putting a safe emotional distance between herself and others. Rei's love has been rejected in the past, and she is afraid of getting burned again.
It must be noted that Rei's mental hangups in no way affect her effectiveness as a Sailor senshi. As Sailormars, Rei is all business and fierce practicality.
Her persona, Sailormars, is the holder of the star seed of planet Mars. The kanji for Mars mean "fire star" in Japanese, hence Sailormars' fire attacks. Mars was the ancient Roman god of war, which relates nicely to Rei's interest in the military arts. Her otherworldly skills as a priestess help her in her fight to protect the Moon Princess. I've read that "Rei" can become "spirit" or "beauty," and "Hino" translates to "of the fire." Sailormars is definitely one of the more effective senshi. Her fiery attacks and her link to the spiritual world combine to make her a deadly fighter, and her attacks are quite often fatal of themselves.
There are two ravens who are very connected to her, named Deimos and Phobos. (These are, quite coincidentally, also the names of the two moons of the planet Mars.) They seem to hold much of her power, and help her when they can. I suppose they could be familiars, of a sort. She met them at the temple when she was very small, and they have shared a close bond since. When the Amazoness Quartet trapped Rei in the mirror house and drained her confidence, it was Phobos and Deimos (in winged-girl form) who came to her and boosted her to a new level of power. It later turns out that during the Silver Millenium, Phobos and Deimos were twin sprites from the planet Coronis, chosen to train and guard Princess Mars in her growing power.
For a closer look at Mars, visit the Fire Senshi's Realm.