There was some great insight already given here by Tracy, katharina, Bry and Kasie. Let me just add that to me, it’s a letter of Balance!
It hides in plain sight in the archetypical Yin / Yang illustration:
So we can associate it with flow, the element Water, (also as mentioned before the Snake 
and the Sacral chakra), the symbol of infinity, 
the triskele 
, the rune Sowilo 
and other symbols of creation and creativity.
We will add a new printable page for the Letter S later this month (after letter R which is coming out this week!). So I’ll make sure to post it here.