Zenos yae Galvus is the son of emperor Varis zos Galvus and Legatus of the XIIth Legion. He serves as the Imperial Viceroy of Ala Mhigo, looking for battles to participate in for fun and sword collection.
So when did Spiritus call Zenos yae Galvus to World B? Sometime after he slits his own throat with the Ame no Habaki. However, rumors persist that he may still be alive. That is, sometime after he allows Spiritus to create a crystal doppelganger for him, he returns to Eorzea.