Achieve

class Game_System

  attr_accessor :achievements

  alias :um_initialize :initialize unless $@
  def initialize()
    um_initialize()
    @achievements = []
  end

  def achieved?( id )
    return @achievements.include?( id )
  end

  def add_achievement( id )
    @achievements << id
    @achievements.uniq!()
  end

end

$game_system.add_achievement( 1 )
$game_system.achieved?( 1 ) # => true