single_character_attribute.rb

class SingleCharacterAttribtue
  attr :my_single_character_attribute

  def my_single_character_attribute = single_character
    raise ArgumentError, "Expected single character, got #{single_character.length} characters." unless single_character.length = 1
    @mysingle_character_attribute = single_character
  end
end

# usage

sca = SingleCharacterAttribute.new
puts sca.my_single_character_attribute #=> nil

sca.my_single_character_attribute = ?y # the question mark operator as used here makes the following character a single character string 
puts sca.my_single_character_attribute #=> y