X by Athena-verse would represent undeterminable Sync ratio. While it can represent ratios beyond S... it can also represent terribad ratio way below C.
Houki is technically an X-rank in Athena but even that was brief and off-screen, when she transitioned from the Uchigane to the Akatsubaki. After that she achieves an S rank as per canon.
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"You cannot be bad at watching a movie. You cannot be bad at listening to an album.
...But you can be bad at playing a videogame, and the game will punish you and deny you access to the rest of the videogame."
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