Rei Hino
Rei is a headstrong girl with a major temper. She's a devoted Shinto, and therefore has powers that relate to that religion, such as receiving visions and being able to "read" fires. She lives with her old, hentai grandfather in the Hikawa Jinja ("jinja" meaning "shrine"). Some of Rei's other interests outside of her Shinto life are composing songs and singing, proving that she is musically inclined (though this is really only addressed in one R episode). Rei attends TA Private Girls' School, which is a Catholic girls-only school. The other girls at TA worship her, even though oddly enough, she is also rejected as being a weird shrine maiden with strange powers.
Rei becomes first involved with Usagi and Ami when buses leaving the shrine reportedly vanish mysteriously, leading worried parents to believe that the weird family in charge of the shrine has kidnapped their children. Jadeite, in disguise, works at the shrine and is the one who is responsible for the students' disappearances. When Rei confronts him, he sends her to some hole in space (or whatever) where he has kept all the other buses. Usagi arrives at the same place when she manages to grab hold of a bus being sucked into a portal. While there, Luna sees the sign of Mars on Rei's forehead glow as she is being held by a youma. After Sailor Moon frees Rei, Luna she gives Rei her henshin stick. Rei then transforms, becoming Sailor Mars, and she destroys the youma of the day with her "Fire Soul". Afterwards, she joins the Sailor Senshi.
Most of the conflict in the first season comes from the constant bickering between Rei and Usagi and also Rei's relationship with Mamoru (the two are dating). Usagi is jealous of this, though she never admits it. To briefly address the relationship, Rei only seems to have a crush on Mamoru, more excited over the idea of having a hot boyfriend than anything else, though admittedly she does feel bad when she discovers that Mamoru and Usagi were lovers in their previous lives, but she deals with it.
Rei's other "love interest" is the guy who eventually works at the shrine late in the first season, Yuuichirou, who has a huge crush on her. Rei won't admit that there's anything between them, though it's pretty obvious she has feelings for him.
Though Rei and Usagi always fight, Rei is the most protective of the Inner Senshi over her. For example, Mars is the one who saves Moon's life on their way to fight Queen Beryl by destroying the last of the DD monsters who was going to kill Moon. Also, in the R movie, Mars is the one who pushes Moon out of the way of the flower-creatures' attack, which took the rest of the Inner Senshi hostage. Rei/Mars often displays "tough love" for Usagi.
One of Rei's other Shinto related powers is her anti-evil scroll power. She throws her ofuda and says, "Rin, pyou, tou, sha, kai, jin, retsu, sai, zen. Akuryou taisan!" This basically means, "Evil spirits, begone!" and it is used to, obviously, get rid of evil. For instance, in the R movie, she throws a bunch of ofuda at people possessed by plants, and they all become unpossessed.