Luna

Luna first appears in the first episode or act of every version of the series. In the Silver Millennium, she was a servant to Princess Serenity, sworn never to leave her side, as well as an advisor to the Queen. When the kingdom fell, she and Artemis were put into a long sleep and sent down to Earth to look after the Senshi, who would eventually be reborn there. Part of Luna's memory was suppressed, in order to make her task easier; she only knew that she was supposed to find and awaken the Guardian Senshi. She found Usagi Tsukino first and taught her to become Sailor Moon, not knowing that this was merely a disguise meant to hide Usagi's true identity as the reincarnated princess.
Over the course of the series, Luna develops a close bond with Usagi, though early on it is on uneasy terms, as Luna often upsets Usagi by giving her advice she does not want, often leading to comic results. She also becomes good friends with Ami Mizuno. She and Artemis have an implied romantic relationship, which is confirmed when they meet Diana, who is their daughter from the future. During the Sailor Stars anime, Luna also develops a crush on Kou Yaten, one of the Three Lights.
Luna has a special manga story devoted to her, entitled "The Lover of Princess Kaguya," in which she falls in love with a human named Kakeru. This story was adapted into the Sailor Moon S movie, and features Luna's first transformation into human form.
Luna has blue eyes in the manga and live-action series, but red eyes in the anime.