Why doesn’t ToUpper() return “*” when applied to “8”?

No, it shouldn’t. ToUpper() doesn’t mean WithShiftKeyOnAnInternationalASCIIKeyboard(). There isn’t an uppercase 8, as 8 is a number, not a letter.

Of course, this is a gross over-simplification (being a number alone doesn’t automatically make a certain character in a character set caseless), but it’s likely what you’re asking for anyway so I’ll leave it at that.