After the Sailor Guardians defeated Nemesis and saved Crystal Tokyo, Chibiusa became Sailor Chibi Moon. So Neo Queen Serenity decided to send Chibiusa to the past for her training, and she sent you to watch over your sister.

So here you and Chibiusa are, walking to the shrine with Diana with you guys.

Chibiusa: I can't wait to see everyone again, especially Mamo-Chan