• Random tests make no sense in this kata, there are only 8 scale degrees anyway.
• Actually, the entire kata does not make sense. Each octave contains the key G twice? Or does each octave start on a different degree?

added.

damn girl, to many bugs

ok

Even there (in Solutions page), you should use spoiler flag, because the comments there are visible in the homepage if not.

Please don't post your solution in the Discourse page, it doesn't add anything since it will be marked with the spoiler flag anyway, and it prevents other users from finding helpful comments by clogging up the page.

You can discuss your (and other people's) solutions in the Solutions page, which is only visible to people who have already solved the kata.

You can pass some tests if you provide array with decimal codes od letters.

instead of saying 'Smaller than two" I would prefer smaller or equal to one, because that confused me a bit :)

JS Test Case:

Test.assertSimilar(evenChars ("a"), "invalid string")
Test.assertNotSimilar(evenChars ("a"), "a")

What's the second test for? If the first failed, the second will as well.

In this case y should be considered as a vowel (you have to make the liaison, for example in les yeux, "the eyes") , but I don't know what tests expects.
EDIT: if tests are complete, they exclude y from vowels.

Fixed

• Node 12 should be enabled (Refer this and this for more info)

The problem isn't because you modify the string, you didn't. The problem is you're creating a standalone function instead of adding a method to the String prototype.
You can take a look at this: MDN: Object Prototypes

Description should define what is supposed to be considered a syllab here (syllabification in English is far from being a trivial question in general, and this is supposed to be a programming challenge, not a linguistics exam; I guess the kata only tests obvious cases, but anyhow something is missing).