Zero takes place at least two centuries after PSO, but it takes place on planet Coral, the origin of the Pioneer civilisations. Mother Trinity (a project provably evolved from the three super computers studied in Ragol: Vol Opt, Olga and Calus) was giving advice to Coralians to save the environment, but politics this and that they didn't listen, so Mother Trinity Photon Erased them to the bone. Two centuries later the survivors in bunkers started life again in the surface, not even knowing their planet was named Coral and called it "Earth". The story only hints the player character as being somewhat similar to someone that fought against Mother Trinity two centuries ago. That only makes sense if your character is a CAST.
@BB-Zero-Four said in Relation with the original games:
I'm not saying anything with this, I'm just throwing it in the ring. I personally only regard Phantasy Star, Phantasy Star II, Phantasy Star IV, Phantasy Star Online and Phantasy Star Universe as being... true Phantasy Star games. Yeah I played III, Portable, Zero, etc, but I kinda see those as extended universe "star wars novels" stuff that don't really carry the same weight as the main titles. In my mind at least. So with that said, I dunno, PSO2 doesn't seem to have the same soul stitched into it as former main PS titles did. It's a cool game but yeah, Alys, Rutsu, Rico, etc all seem much more fleshed out and 'Phatasy Stary' to me than anything I've seen in PSO2. That's just one man's opinion. And an old man at that.
I'm stumped as "an old man" you recognise PSU but not Portable. I mean, most nostalgic phantasy star players I've met just discard PSU entirely, and most will agree PSO is a completely different thing than the classic games, AND the only way it could get linked to the series is precisely one of 4 endings from the classic game you don't consider a "phantasy star" (the one that introduced rappies, btw).