'ẞ' LATIN CAPTIAL LETTER SHARP S (U+1E9E)
It is the uppercase variant of U+00DF LATIN SMALL LETTER SHARP S, which is widely used in Germany (comparable to any other letter in the German alphabet). The uppercase variant has seen lower usage today, as the ß is often replaced by SS in uppercase writings due to historic circumstances or used in combination with uppercase letters where such a substitution would lead to ambiguities like in names.
This letter is necessary for the faithful representation of German texts as old as 130 years and improves typography quite a lot by obsoleting mixed-case writings like "CAFE SCHLOßSTEG".
The German glyph LATIN CAPTIAL LETTER SHARP S (U+1E9E) is currently not in the Ubuntu Font Family. Other free fonts frequently used in open-source contexts like DejaVu or Linux Libertine as well as the fonts of other recent operating systems have it, so it would be nice if the Ubuntu fonts have this character eventually.