Sailor Moon (セーラームーン, Sērā Mūn?) is a fictional character in the Sailor Moon metaseries and the main protagonist of the franchise, as well as its title character. Her civilian name is Usagi Tsukino (月野 うさぎ, Tsukino Usagi?, Serena Tsukino in the English versions). She is a carefree schoolgirl who can transform into the de facto leader of the series' primary heroines, the Sailor Senshi.

Due to the series' widespread popularity in many countries, as well as her distinctive and oft-copied odango hairstyle, she is one of the most immediately recognizable and iconic anime characters worldwide. She is the only character to appear in all 200 episodes of the anime, and is present in all 52 acts of the manga as well as all 51 acts of the live-action series.

Serena, Usagi's given name in the English versions of the series, is derived from "Serenity," the name Usagi went by in her past life. In the English manga she is mostly called by the nickname Bunny, which is the literal meaning of usagi. Her boyfriend for most of the series, Mamoru Chiba, calls her by the affectionate nickname "Usako" (which is a diminutive, the suffix -ko meaning "child").

Source: Wikipedia