According to comments on post #1613467, Satsuki went from smiling and bubbly to being withdrawn and reserved as early as age 5.
Didn't it say in the show that Her Father Told her the truth about her mother, the baby sister that never got a chance to be, and the plans for the world with the Life Fiber
When she was 5
assuming then She turned sour and was already plotting her mothers downfall.
a theory i once heard was that iori satsuki and nonon were playing with nonon's monkey until the monkey lands in the vicinity of nui. she finds no interest in the monkey so she treats it harshly and throws it in a river/pond. iori tries to to go to the river/pond to get the monkey back and satsuki tries to retaliate against nui both matters unsuccessfully dealt with. this is why iori is wet, nonon is crying, and satsuki is holding both their hands looking very upset for failing them. this could also be why nonon's uniform has a monkey bones as a memento from iori to nonon.
