`b` A hidden method for Ruby String

In irb console, I have this code:

class Abc
  attr_accessor :a, :b

  def self.new(a, b)
    @a = a
    @b = b
  end
end

o = Abc.new("a", "b")

o.a  # => raise error Nomethod
o.b  # => return string "b"

Why? why o.a raise error but o.b return string b????

Let’s see new method. We add @b = b, so this method returns b. So o = Abc.new("a", "b") => o = "b". And o.b ~= String "b".b

So I find here documment for b method :v

Ruby support a method to returns a copied string whose encoding is ASCII-8BIT.

>> checkmark = "\u2713"
=> "✓"
>> checkmark.b
=> "\xE2\x9C\x93"
>>

So, no magic here, It is just plain Ruby and we just found a mysterious method String#b