How can I convert a single Character type to uppercase?

To call the C toupper() you need to get the Unicode code point of the Character. But Character has no method for getting its code point (a Character may consist of multiple code points), so you have to convert the Character into a String to obtain any of its code points.

So you really have to convert to String to get anywhere. Unless you store the character as a UnicodeScalar instead of a Character. In this case you can do this:

assert(unicodeScalar.isASCII) // toupper argument must be "representable as an unsigned char"
let uppercase = UnicodeScalar(toupper(CInt(unicodeScalar.value)))

But this isn't really more readable than simply using String:

let uppercase = Character(String(character).uppercased())