Sailor Mars, in the person of Rei, has just made her appearance at "Sailor Moon Crystal." Her awakening came as a most pleasant surprise because instead of being just another victim rescued by the sailors, she herself emerged as the surprise heroine and ended up saving not just the victims, but the sailors who came to their aid as well.
As to how she eventually found herself woven into the Sailor's saga, Den of Geek has this gist of episode 3 to share:
People speculated that a dubious bus that was last seen at the Hikawa shrine was responsible for the string of mysterious disappearances. Hence, they started calling it as the "Demon" Bus. Its presence at the aforementioned shrine came to Rei Hino's attention. A miko at the Hikawa shrine, she is possessed with psychic abilities. With the help of her visions, Rei found Jadeite who was the mastermind behind the cryptic disappearances.
However, the villain captures her and keeps her prisoner, along with the other victims in the Dark Kingdom. "Sailor Moon Crystal" members Sailor Mercury and Sailor Moon enter in an attempt to save them, but they failed. At this point, Rei's identity as Sailor Mars emerges, and with her newly awakened powers, defeats Jadeite, thus saving everyone.
According to IGN, Sailor Mars' characterization was that of an ill-tempered, sharp-tongued girl who was both friend and enemy to Usagi. But the site noted that in "Sailor Moon Crystal," her feistiness has been toned down conspicuously.
Den of Geek observed that episode 3 stayed true to the origin tale of Sailor Mars. They commented that this was a necessary move because all of them are just in the process of being introduced. Each debut features an interesting and amusing adventure that matches the character's development.
In comparing the three episodes, the third one, according to Den of Geek, strayed from the manga. Although the deviation did not produce much difference, it was enough to improve it. With the plotline of "Sailor Moon Crystal" becoming more manga in overall tone and characterization, the Sailor Mars introduced in this episode seems to follow the manga version.
