User blog comment:Hawkeye2701/Kubo Lost Count/@comment-5398287-20141109155813/@comment-5398287-20141110142739

As far as we know, not all Sternritter require a letter. Given that both Yhwach and Uryu have a letter and are not Sternritter, we know that there can be non-Ritter individuals with letters, so it stands to reason that the opposite is also true.