Shizuku is the reason for the travels and journeys the girls have had. Shizuku can be recognized by light green hair and her only article of clothing is a poncho. She is portrayed as a goddess of the dandelions. For most of the anime she is referred to by the three sisters as "The One" as they do not know her name.

Spoiler

When the sisters finally meet Shizaku, she gives them a test to allow them to go back to an era when they were happiest or continue to travel.

(Source: Wikipedia)