Rei's history is largely the same across the different versions of the story. She works as a miko or shrine maiden at the Hikawa Shrine , and is shown to have an affinity with two crows who live there. It is revealed in the manga that as a child, they "told" her that their names are Phobos and Deimos (the same as Mars' two moons). In the manga she is portrayed as calm, serious, and practical, distrusting most men and discouraging her friends from developing romantic relationships. In the other adaptations, though, her personality is quite different.

In the anime, Rei is fiery, boy-crazy, "down-to-earth" and ambitious, longing to one day become a singer, model, and voice actress envisioning herself as ultimately getting married. She is also greatly interested in pop culture and shows musical talent of her own, including playing the piano and guitar, singing, and composing all the songs for a school festival. Rei frequently engages in petty arguments with Usagi, who she sees as immature, and although she becomes calmer as the series progresses, remains somewhat more of a typical teenager than her manga counterpart. Anime Rei also tends to engage in long periods of sticking her tongue out at Usagi (who returns the favor), as a kind of running gag in their arguments.

Rei goes to a different school from the other girls, namely T*A Private Girls School, a Catholic institution run by nuns. She herself is a practitioner of Shinto, living and working at Hikawa Shrine with her grandfather, its head priest. Her mother died when Rei was very young; her father is a high-class and famous politician who cares more about his job than about her (though in the live-action version he still tries to be involved in her life), and who only visits Rei on her birthday. She carries a certain amount of bitterness toward him, especially in the live-action series, in which the character of her grandfather does not appear.

Other characters frequently comment on Rei's beauty and elegance—in her first appearance, these traits even lead Luna to suspect that she could be the Princess they have been searching for. However, her strange psychic talents have caused some to be afraid of her, and her distant nature makes her unreceptive to close friendship. Rei does treasure the friends she has, which consist entirely of the other Senshi, and although she sometimes adopts an aloof, big-sisterly attitude around them, this façade is frequently broken by her own enthusiasm, her genuine affection for all of them, and her sharp tongue.

Because of the lack of respectable males in her life, the manga and live-action Rei harbors a generally low opinion of all men. She considers them emotionally weak and untrustworthy and seems genuinely uninterested in romance. The one exception is in a manga side-story centering around her, Casablanca Memories, which tells of Rei's friendship with her father's young secretary, Kaidou. He had been kind to her for her entire life and, in the story, she fancies herself in love with him. She is shocked when he suddenly announces his engagement to another girl and his decision to become a politician, despite having once said that he didn't like what had happened to Rei's family as a result of her father's work. Proof of her feelings are further cemented when she moves to kiss Kaidou, asking why, if he wanted to marry into the profession, didn't he choose to marry her due to her father's political influence. In the manga, this is the only potential romance in her life; in the live-action series, nothing of the sort is ever shown, as Rei is pointedly disinterested in boys. In the anime, however, she dates Mamoru Chiba for a short while before he and Usagi begin dating, and occasionally seems open to a relationship with Yūichirō, her grandfather's pupil. In one episode, on the brink of death, she states that she wishes she had kissed Yūichirō before leaving.

Late in the manga, members of the Dead Moon Circus harass Rei with a reflection of her young self, which mocks her for her friendships and her dream of being a priestess. The reflection tells her that the only way for her to be happy is to try her luck with numerous men until she ends up married to someone rich. Rei is able to defeat this illusion, and in the process gains her Sailor Crystal along with the memory that, long ago, she had in fact made a vow of chastity to Princess Serenity. After this realization, she is never again shown having any doubts about her lack of interest in romance. This is never mentioned in the other series, although in the equivalent anime episode she expresses an intention to remain single forever.

Rei's lifelong dream is to become the head priestess at Hikawa Shrine, and much of her life is influenced by spirituality, particularly in the manga. Meditation is given as her strong point, and she enjoys fortune-telling as a hobby. The elegance of her character is further underscored by the contrast between her favorite subject, ancient writing, and her least favorite, modern society. She also belongs to the Archery club at school, which later provides the context for her most powerful weapon, the Mars Arrow. As for more general tastes, it is mentioned in the manga that Rei enjoys fugu, a type of blowfish which is both highly toxic and considered a delicacy in Japan. Other favorite things include pandas and the colors red and black; dislikes include canned asparagus, men in general, and television. The manga states that she does enjoy Devilman, which the English manga changes to Buffy.

Just as Rei is more fiery in the anime than in the manga, she is changed again in the dubbed series. Although shots of her actually slapping Usagi are removed, remarks about her childishness and unreliability are more frequent and often harsher. In episode 43 of the original, Sailor Mars is briefly entrusted with the Moon Stick, which she explains to the others as proof that she and Sailor Moon don't hate each other. In the dubbed equivalent, she says that Sailor Moon left it at her house by accident, and that she took it because she believes she would make a better leader. Other episodes add dialogue in which "Raye" tells people off or is rude to them, where in the original she is only mildly frustrated.

Though Rei and Usagi's arguments are a constant in the anime series, it is shown several times that they are extremely close friends and care deeply for each other. Rei is the last Senshi to die in the battle against the DD Girls in the first season's finale, giving her life for Usagi. In Sailor Moon R: The Movie, Rei is hysterical when Usagi gives her own life for her friends. When Sailor Galaxia kills all the Guardian Senshi in Sailor Stars, it is Sailor Mars to whom Sailor Moon immediately rushes. In the manga, Rei is Usagi's confidante and the first one she goes to with problems, and vice versa.

In the live-action series, Rei's personality is based more on the manga than on the anime. She has a difficult time trusting people, even her fellow Senshi, and has a tendency to rely too much on herself. She claims to hate karaoke (which the other girls love, especially Usagi), but is later coerced by Minako into posing as an idol—exactly what her anime counterpart wishes. Under the name "Mars Reiko," she appears on three occasions: performing for hospitalized children, working alongside Minako, and staging a contest with Minako in order to stop her from quitting the idol business. She has a complex relationship with Minako; though they often disagree and compete with each other, they also share great respect. Minako even sometimes lets her guard down around Rei, and eventually confides to her about what she feels is her destiny as a Senshi. Rei is told that she is to be the secondary leader of the Senshi (as in the manga), and alternately admires and resents Minako's teachings about what that means.

Special powers and items Rei is one of few Sailor Moon characters who is able to use special powers in her everyday civilian form. These are mainly the result of her role as a miko, which gives her heightened spirituality as well as certain resources. In addition to some psychic talent, including occasional unprompted premonitions, Rei is able to do fire readings and to dispel evil spirits. She does the latter by performing Kuji-Goshin-Ho, a ritual which consists of chanting nine words of power (rin, pyou, tou, sha, kai, jin, retsu, zai, zen) while making relevant hand signs. She then shouts "Akuryō Taisan!" , meaning "Evil spirits, begone!" and throws an ofuda scroll (or several). She commonly uses this attack while in her Sailor Mars form as well as when she is in her civilian form.

Rei must transform into a Sailor Senshi, however, before she can access her celestial powers. She makes this change by raising a special device (pen, bracelet, wand, or crystal) into the air and shouting a special phrase, originally "Mars Power, Make-up!" As she becomes more powerful and obtains new transformation devices, this phrase changes to evoke Mars Star, Planet, or Crystal Power. In the anime, Sailor Mars' transformation sequence evolves slightly over time, whether to update the background images or to accommodate changes to her uniform or new transformation items, but they all involve rings of fire that circle her body as she spins, forming her outfit in a flash of light.

Sailor Mars has the power to manipulate flame. Her named powers are somewhat inconsistent across the various series—in the first arc of the manga, she says "Akuryō Taisan," the same phrase she uses as a civilian while using an ofuda, for her fire attacks. In the anime, she shoots a fireball from her index fingers and shouts the words "Fire Soul" and in the live-action series she shouts "Youma Taisan" ("Monster, Begone"). This basic power is improved to "Fire Soul Bird" in the second arc of the anime only; the manga also has an animal-based attack, "Mars Snake Fire," but it does not appear until the third story arc. Her first attack to be the same across all versions is "Burning Mandala," which incorporates Buddhist symbolism in the fiery rings Sailor Mars summons. She is apparently not immune to her own powers, as she is able to use them for a suicide move in the anime.

Sailor Mars' final and most powerful power comes in the fourth story arc, when she takes on her second Senshi form (Super Sailor Mars in the anime). At this point in the series, she acquires a special weapon, the Mars Arrow, and with it "Mars Flame Sniper," which is her primary attack for the duration of the series. In the manga, the Mars Crystal and Mars Arrow are among her most significant mystical possessions. The former is her Sailor Crystal and the source of all of her power. The latter is associated with her skill as an archer, and although she does not receive a physical bow, she recalls some advice given to her by Michiru Kaioh—"[If] you think you're being taken advantage of by the enemies, stretch a line taut in your soul. Then, with your whole body and spirit, shoot the arrow of your finishing blow!" In the live-action series, she is given a tambourine-like weapon, called the Sailor Star Tambo, by Artemis. During the final battle, her Tambo transforms into a dagger, which she uses alongside Venus's similar dagger. In the "Special Act", Venus wields both weapons.