Basically this entire thing is a spoiler for multiple books.
So during a reread of Dark Age I noticed that Lysander au Lune seems to go on for quite a while about his relationship with Ajax au Grimmus, the son of Aja au Grimmus and Atlas au Raa. Seeing that Lysander was essentially a surrogate son of Aja, Lysander and Ajax are supposed to have been best friends growing up. Lysander speaks of Ajax as a "kind, generous brother who could never bridle either affection or fury," (p.30) and both seem to be overjoyed when they are reunited.
Assuming Ajax and Lysander are around the same age, Ajax should have been kept mostly out of the way, like Lysander. Not off on campaigns with the Ash Lord or some other Imperator, right? Being the product of a "loveless genetic union", Ajax was likely being raised to be the Sovereign's next apex killer, so she'd want to keep him safe before he could kill things really well. The safest place was with her and Aja, where she kept Lysander.
Specifically where he should have shown up is either during Golden Son, when the Sovereign attempts to bring Darrow to her side, or later (I think in Morning Star) when Mustang is reliving her time on Luna with Cassius. Both of those times mention Lysander in detail, and shouldn't Lysander's best friend have been running around with him? Or maybe Ajax could have been a small, dark shadow hiding behind Aja?
So why is he never mentioned in the first three books? Or even in Iron Gold? Pierce very rarely (if ever) makes continuity mistakes, so it wouldn't make sense that he would just make up Ajax for the sake of book five. He's really a cool character and absolutely makes sense with the kind of stuff the Sovereign would do, so any theories as to why he isn't mentioned until he's useful?