S is the only consonant that survives a whisper completely. Try it: whisper a word with B in it, the B falls off, you get an unvoiced p-shape with no plosion. Whisper a word with S in it and the S is still S, exactly as loud as itself. It doesn't need the voice. It was already breath.
Shape: Phoenician shin meant tooth. You can see it if you draw the original — three points, sharp, the bite-mark. The modern S is shin softened by twenty-eight centuries of cursive hands rounding the corners off the bite. It became a curve because curves are easier to write fast. The snake-resemblance is incidental. The snake-sound was there first.
The long s — ſ — was lowercase S in the middle of words until about 1800. It looked like an f without the crossbar, or with half a crossbar, depending on the font. You can read eighteenth-century books where Congress is written Congreſs and it looks, to a modern eye, like Congrefs. The letter was retired by typographers without ceremony. Sometime around 1810 you stop seeing it. Nobody mourned. A whole letterform — one of the most common ones in English, used millions of times a day — just stopped existing. People kept reading.
S is the most generative letter in English because it makes things plural. Every noun extension, every verb conjugation in the third person, the possessive. You cannot write a paragraph in English without it. It does more grammatical work than any single letter has any right to do, and it does the work quietly, attached to the end of something else, never the subject.
Ranked fourth in the letters ranked. That was too low. Moving it up.