[quote=@Fractured]
<Snipped quote by DarkwolfX37>

Is Codi the plural of Codex? I had always assumed it was its own plural.
[/quote]

Codi is the species that Codex come from, barring the ones after Yander's death when you guys had non-Codi become Codex. They're also tied with three others as the first species in Existence, since the original Codex were all Codi too. There's also a spoiler-y link between them and humans beyond them just looking similar.