The first was 2003's Saint Seiya: Episode.G by Megumu Okada, of Shadow Skill fame, which was a prequel starring Leo Aiolia & has since become a massive franchise of its own, with two sequels (Assassin & Requiem) totaling 43 volumes across all three parts (as of this review, at least) & even a couple of drama CDs. Then in 2006 came Saint Seiya: The Lost Canvas by Shiori Teshirogi, a former assistant for Kozue Amano during Aria's serialization, which detailed an alternate universe version of the Holy War between Athena & Hades' forces in the 1700s (due to Saint Seiya: Next Dimension telling its own story in that same time period that supersedes LC) & would become the most recognizable of the Seiya spin-offs, running until 2011 for 25 volumes before immediately getting a side-story Gaiden series of its own that'd run until 2016 for another 16 volumes (41, in total), & from 2009 to 2011 would even get a 24-episode OVA adaptation by TMS that adapted up to just shy of the halfway point. There was also a short-lived manga version of Saint Seiya Omega by Bau in 2013 that only made it to one volume before the magazine it ran in (Kerokero Ace) got canned. Then there's 2020's Saint Seiya: Dark Wing by Kenji Saito (writer) & Shinsu Ueda (artist), which is an isekai story where a high school student & his classmates & teachers get reincarnated as the forces of Athena & Hades in a completely original plot from the OG series. Most recent of them all is the 2022 duo of Saint Seiya: Rerise of Poseidon by Tsunakan Suda, a midquel taking place during the Hades Chapter's latter half & sees Poseidon & his revived Mariners needing to protect the world from the forces of Nemesis, & Saint Seiya: Time Odyssey by Jérôme Alquié (writer/artist) & Arnaud Dollen (co-writer), which sees the OG cast take on the forces of Chronos. However, there's one spin-off manga that I didn't mention here, and that's because it's the only one to have received a complete official English release (since Ablaze is still currently releasing Time Odyssey, which itself it still ongoing).
Debuting in the pages of Akita Shoten's Champion Red magazine in mid-2013 (specifically two issues after Episode.G had ended), Saint Seiya: Saintia Sho was done by Chimaki Kuori, who at the time was known primarily for being the illustrator for a variety of light novels (Parsley Legend, Rumble Fish, Ambition Waltz, Mobile Suit Gundam: High Streamer, etc.) & for making the manga Gundam Seed Destiny: The Edge, which retold the events of the 2004 anime from the perspective of Athrun Zala. Not just that, but Saintia Sho itself was notably different from other Seiya spin-off manga of the time because it actively took place during the main plot of the original manga, instead of taking place either before the OG manga or in its own separate timeline; again, Rerise of Poseidon & Time Odyssey came later on. Not just that, but this was also a female-focused series, introducing the concept of Saintia, which acted as Athena's personal guard of handmaidens. Finally, though admittedly the most minor of all, the title itself (& the name of the main character) was a direct reference to Silent Knight Sho (right down the literal kanji for "翔/Sho"), the short-lived failure of a 1992 manga Kurumada made after Saint Seiya. Saintia Sho would run until mid-2021 & total 16 volumes (as well as receive a 10-episode ONA adaptation in late 2018 that was generally not liked by fans), but in 2017 Seven Seas announced that it had licensed the manga, the first time a manga with Masami Kurumada's name on it would receive an official English release since Viz & TokyoPop both finished their releases of Saint Seiya & B't X, respectively, in 2010.
Unfortunately, despite trying to initially market it to fans of Sailor Moon, Seven Seas' release of Saintia Sho seemed to bomb just as hard as Viz & TokyoPop's attempts at releasing Kurumada manga in English did back in the 00s. While there are no sales numbers to reflect this, it is telling that Seven Seas stopped printing the opening splash pages in color after Volume 9's release (i.e. it was no longer worth the extra cost to do that, though these pages were actually B&W in the original Japanese tankouban), the release schedule was admittedly never all that consistent to begin with (even when there was no worry about catching up to Japan early on), and Volume 15's physical release kept getting delayed after its digital release that once it finally came out the final volume would come out literally just a week later(!); also Seven Seas doesn't even bother to sell it on their own webstore, which I think says it all. Regardless, Seven Seas still managed to release all of Saint Seiya: Saintia Sho, releasing the final volume in January of 2023 (five years, faster than Viz or TokyoPop!), so as part of this blog's year-long celebration of Masami Kurumada's 50th Anniversary let's see if this spin-off was truly the right one to release in English for newcomers and if it's a good series, in & of itself.