Devil Saber is a fusion between Sakura Saber and Demon Archer. She makes it her mission to put an end to the Fate franchise in the Koha-Ace World. This backstory is later changed for her more serious incarnation in Fate/KOHA-ACE.
Devil Saber appears as a young woman with long white hair, tied back at either side with red tassels. She wears a red and black dress bearing similarity to a short kimono and long European style coat, complete with high collar held closed by a rope tassel. She appears to wear long stockings.
Various characters try to defeat her with no effect, including Saber, Mordred, Lancelot, and Gawain. She is eventually defeated by Arcueid Brunestud, Aoko Aozaki, Nagato from Kantai Collection, a myriad of other characters, and Saber.
After XP was finished, the author wanted to utilize the design more, so he thought "I'll somehow get it out there... Yeah! I'll use it as something like Okita Alter in Imperial Capital!" and utilized her in Fate/Koha Ace. As for Demon Archer's part in it, "The Nobu's component? Who's that? Someone who rolls the gatcha?" When asked why her background was changed, she says it's so she can appear in Fate/Grand Order.
Saber has a great advantage over characters representing the Fate franchise.