Below are all codepoints supported by Athena Ruby, with standard Unicode in the first half and nonstandard in the bottom.
, . / : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [ \ ] a b c d e f g h i j k l m n o p q r s t u v w x y z | · 𐆂 𐆄 ˁ ˊ ˋ ˙ ̀ ́ ̄ ̅ ̆ ̈ ̓ ̔ ̣ ̷ ͂ ͞ ʹ ͵ ΄ ΅ Ά · Έ Ή Ί Ό Ύ Ώ ΐ Α Β Γ Δ Ε Ζ Η Θ Ι Κ Λ Μ Ν Ξ Ο Π Ρ Σ Τ Υ Φ Χ Ψ Ω Ϊ Ϋ ά έ ή ί ΰ α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω ϊ ϋ Ϗ ϗ Ϙ ϙ Ϛ ϛ Ϡ ϡ 𐆍 𐆎 ἀ ἁ ἂ ἃ ἄ ἅ ἆ ἇ Ἀ Ἁ Ἂ Ἃ Ἄ Ἅ Ἆ Ἇ ἐ ἑ ἒ ἓ ἔ ἕ Ἐ Ἑ Ἒ Ἓ Ἔ Ἕ ἠ ἡ ἢ ἣ ἤ ἥ ἦ ἧ Ἠ Ἡ Ἢ Ἣ Ἤ Ἥ Ἦ Ἧ ἰ ἱ ἲ ἳ ἴ ἵ ἶ ἷ Ἰ Ἱ Ἲ Ἳ Ἴ Ἵ Ἶ Ἷ ὀ ὁ ὂ ὃ ὄ ὅ Ὀ Ὁ Ὂ Ὃ Ὄ Ὅ ὐ ὑ ὒ ὓ ὔ ὕ ὖ ὗ Ὑ Ὓ Ὕ Ὗ ὠ ὡ ὢ ὣ ὤ ὥ ὦ ὧ Ὠ Ὡ Ὢ Ὣ Ὤ Ὥ Ὦ Ὧ ὰ ά ὲ έ ὴ ή ὶ ί ὸ ό ὺ ύ ὼ ώ ᾰ ᾱ ᾶ Ᾰ Ᾱ Ὰ Ά ᾽ ῀ ῆ Ὲ Έ Ὴ Ή ῍ ῎ ῏ ῐ ῑ ῒ ΐ ῖ ῗ Ῐ Ῑ Ὶ Ί ῝ ῞ ῟ ῠ ῡ ῢ ΰ ῤ ῥ ῦ ῧ Ῠ Ῡ Ὺ Ύ Ῥ ῭ ΅ ` ῶ Ὸ Ό Ὼ Ώ ´ ῾ ↀ ◌ ☧ ✚ ⸉ ⸊ ⸣ ⹄
Ligature test: ΑΒ ΓΩ ΑΒ ΛΙ
Variant-one: ABCDEFGHIJKLMNOPQRSTUVWXYZ ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ
Variant-two: ABCDEFGHIJKLMNOPQRSTUVWXYZ ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ
Variant-three: ABCDEFGHIJKLMNOPQRSTUVWXYZ ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ
Variant-four: ABCDEFGHIJKLMNOPQRSTUVWXYZ ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ
Variant ligature test: ΗΡ ΗΡ ΗΡ
Contextual alternate test: A̅ B̅ C̅D̅
Variant contextual alternate test: pending
Not the optimal method for accessing the character variant you're looking for, but it's there if you need it.
Stacking decoration: