Usagi Tsukino (月野 うさぎ, Tsukino Usagi), better known as Sailor Moon (セーラームーン, Sērā Mūn), is a fictional superheroine who is also the protagonist and titular character in the Sailor Moon manga series written by Naoko Takeuchi. She is introduced in chapter #1, "Usagi - Sailor Moon", originally published in Japan's Nakayoshi on December 28, 1991, as a carefree schoolgirl who can transform into Sailor Moon, the de facto leader of the Sailor Soldiers. She is initially believed to be an Earthling, but is later revealed to be the princess of the moon kingdom Silver Millennium. She later discovers her birth name, Princess Serenity (プリンセス・セレニティ, Purinsesu Sereniti).