U+0178 Latin Capital Letter Y with Diaeresis

The Wikipedia has the following information about this codepoint:

ÿ is a Latin script character composed of the letter Y and the diaeresis diacritical mark. It occurs in French as a variant of ⟨ï⟩ in a few proper nouns, as in the name of the Parisian suburb of L'Haÿ-les-Roses [la.i le ʁoz] and in the surname of the house of Croÿ [kʁu.i]. It occurs in a few Hungarian names as well, such as Lajos Méhelÿ and Margit Danÿ.

As ⟨ÿ⟩ rarely appears as the first letter in a name, and all-caps text typically omitted all accents, initially there was assumed to be no need for an uppercase ⟨Ÿ⟩ when computer character sets such as CP437 and ISO 8859-1 were designed. However much software assumes that conversion from lower-case to upper-case and then back again is lossless, so ⟨Ÿ⟩ was added to many character sets such as CP1252, ISO 8859-15, and Unicode. This also happened to a more prominent character, the German ß.

IPA uses ⟨ÿ⟩ to transcribe the close central compressed vowel, a type of vowel sound used in some spoken languages.

The character has also found use as a metal umlaut.