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In Sailor Moon, there is a Sailor Senshi (or Sailor Guardian) that represents each planet. The girl in the purple uniform is Sailor Saturn, aka Hotaru Tomoe.

Unlike the other Solar System Senshi, who work together and have responsibilities that are respected, Sailor Saturn is known as the "Forbidden Senshi" whose awakening is feared. This is because her role is to wipe out worlds that have run their course. In other words, she is the pure force of destruction that paves the way for rebirth.

However, Saturn's powers are a double-edged sword: if she drops her Silence Glaive (the weapon she's carrying in the picture) and unleashes destruction, she herself will die, too.

During most of the third arc of Sailor Moon, Sailors Uranus (the other character in the picture), Neptune, and Pluto are desperately trying to prevent Hotaru from awakening as Sailor Saturn because if she does she'll end the world (even to the point of trying to kill her). She ends up awakening during the climax of the arc, but she tells Sailor Moon that her job is a necessary one because it lets Sailor Moon bring hope and rebirth. She sacrifices herself to stop the main villain of the arc. However, Sailor Moon is able to bring her back as a baby so she has a chance to live a better life.