Setsuna

    To the senshi and her enemies, she's known as SailorPluto. In normal life, she's Setsuna. But to ChibiUsa, she's simply Puu.
  Setsuna is introduced in SailorMoon R as the guardian of time. She gave ChibiUsa a time key that allowed her to travel back to 20th century Tokyo and find the Ginzuishou. After the Black Moon family invaded Crystal Tokyo, ChibiUsa went to SailorPluto for help. Setsuna couldn't resist helping Serenity's precious daughter, and instructed her to find the Ginzuishou as anyone might do. However, as time went on, Puu grew affecionately closer to small lady, and made a personal promise that she would guide and protect ChibiUsa as if she were her mother. Puu doesn't lie, as a result, she keeps her vow and constantly watches out for ChibiUsa throughout many of her adventures and heartbreaks.
  There was a subtle turning point in SailorMoon S, when SailorPluto and the other outer senshi recurrently tried to kill SailorSaturn, ChibiUsa's best friend. Pluto's understanding shines through, as she gives ChibiUsa a warm hug as she stands outside of Hotaru's house on a snowy day. Even when ChibiUsa rushes through her house, and doesn't find any trace of Hotaru, she yells,"I hate you, Puu!" In response, Setsuna hugs her, and ChibiUsa forgives.
   Every strong person has a mentor. We know who ChibiUsa's is. :-)