What does the unary question mark (?) operator do?

It returns a single character string. It is the shortest way to write a single-character string literal. Use it when you want to define a lot of single-character strings. It is a heritage from Ruby <1.9, where it used to return the ASCII code for that character. I don't understand what you mean by "break the language orthogonality".

It's not an operator, it's a character literal. However, there is no character type in Ruby, so instead of an instance of a character type the character literal evaluates to the "default representation of a character". In Ruby 1.9+, that's a String of length 1, in Ruby 1.8, it's a Fixnum denoting the Unicode codepoint of the character.