For instance, the letter Sigma (Σ uppercase) has two lowercase variants: ς (it's called "final letter form" and σ. Currently, only σ is supported.

An excerpt from Wikipedia article:

The letter sigma ⟨Σ⟩ has two different lowercase forms in its standard variant, ⟨σ⟩ and ⟨ς⟩, with ⟨ς⟩ being used in word-final position and ⟨σ⟩ elsewhere.[15][18][19] In some 19th-century typesetting, ⟨ς⟩ was also used word-medially at the end of a compound morpheme, e.g. "δυςκατανοήτων", marking the morpheme boundary between "δυς-κατανοήτων" ("difficult to understand"); modern standard practice is to spell "δυσκατανοήτων" with a non-final sigma.[19] The letter sigma also has an alternative variant, the lunate sigma (uppercase Ϲ, lowercase ϲ), which is used in all positions.[15][18][20] This form of the letter developed during the Hellenistic period (323–31 BC) as a simplification of the older Σ σ/ς variant.[20] Thus, the word stasis can either be written στάσις or ϲτάϲιϲ.[21] In modern, edited Greek texts, the lunate sigma typically appears primarily in older typesetting

This part of lib/greek_abc/alphabet.rb doesn't seem very right to me:

module GreekABC
  class Alphabet
    attr_accessor :letters

    ALPHABET.map.with_index(1) do |(name, letter), index|
      define_singleton_method(:"#{name.downcase}") do
        Letter.new(position: index, name: name, lower: letter,
                   upper: letter.upcase)
      end
    end

    def initialize
      @letters = ALPHABET.map.with_index(1) do |(name, letter), index|
        Letter.new(position: index, name: name, lower: letter,
                   upper: letter.upcase)
      end
    end

A nicer way of doing the same thing should be considered.