ẞ

A letter unique to the German language

Fußball, Weißwurst, Scheiße: ß is seen in some of the most internationally renowned German words out there. It ranks 26th out of 30 in letter frequency in German standard texts. However, its upper case version ẞ is likely to be one of the most scarcely used characters in any Indo-European language. It exclusively appears in all-caps notation, as ß is never used at the beginning of a word. 

Speaking of the letter ß in general, it derives from a ligature of the so-called “long-s” (ſ) and a special version of the lower-case z (ʒ)—so in the end, ſʒ simply melted into ß. Its use has changed over time, but as it stands today, the letter indicates a voiceless s after a long vowel or diphthong. To give an example where the letter does a meaningful job: ein weiser Mann (a wise man) goes with a voiced s (as in “zoo”), ein weißer Mann (a white man) goes with a voiceless s (as in “sea”). 

Today, German is the only language in the world that uses this character, making ß as iconically German as Dosenpfand or Mülltrennung. 

References:
https://de.wikipedia.org/wiki/Buchstabenhäufigkeit