S

As a child, s was friends with f, r, i, e, n, and d. He witnessed how f formed the word "frick" along with y, a and o.

Somewhere before F's attack, S turned into an adult.

In R, S was shown on MAP.

S was fully introduced in S in which he was slurped by F right before he finished saying his name.

In Y, F vomits S and chains him up.

In 5, his power is revealed. He accidentally swallows three fragments of Z's moons shortly after they were shattered by P from being launched into space, growing in size. Eventually, he consumes more and more fragments, gradually increasing in size to the point where he is able to stretch across Z's upper surface. At the end of the episode, he is the only letter visible still on Z's surface, now gargantuan in size, still consuming fragments and possibly parts of Z's terrain. S is a Letter who is hardly known to smile, the only time he does so is when he danced with D, E, F, I, N, and R to spell "friends" when they were little and when he ate Z's moon's fragments.