User blog comment:B214/Question and Answers 6/@comment-158.121.180.32-20180928151955/@comment-2601:8C:601:5692:6089:5245:14CC:C305-20180928180951

Wah?!? There's no time-travel! Zero is essentially a prequel story that tells the story of Sirzechs & Grayfia, as well as the civil war between the Maou Faction loyalists, who follow the original Maou's children, and the anti-Maou Noble Faction, led by the House of Baal with Sirzechs, Ajuka, Serafall, and Falbium as their figurehead.

It is similar to High School DxD EX in that it is more of a "novella" with multiple chapters, rather than a short story (which are at best equal to one chapter). As such, it is part of the list of Light Novels rather than in the short stories. However, the wiki has not yet been updated with its information, both because their is still one more chapter yet to be released in Japan, and because no chapters have been translated as of yet.

For spoilers, you can go to this Blog  or here on Animesuki . Other than that, I haven't had much luck finding anything either, even while searching in Japanese. At best, you can get some details from the Japanese Wikipedia characters page, from the part about the children of the Four Maou towards the bottom of the Devils' section. But that info is mostly covered in the spoilers.