WARNING: The following article contains major spoilers for Doomsday Clock #11 by Geoff Johns and Gary Frank, on sale now.
Fans have long known Saturn Girl as a charter member of the original Legion of Super-Heroes. So why has a character from a millennium in the future been seen in present day, starting in DC Comics: Rebirth #1? Geoff Johns and Gary Frank's Geoff Johns and Gary Frank's Doomsday Clock #11 addresses the mystery of her presence in earnest, but another pops up when Saturn Girl disappears.
Saturn Girl Needs to Save Superman
In Doomsday Clock #7, Saturn Girl explained her reasons for being in the present day. She was sent back from her own era to "cleanse the timestream of an unknown anomaly" that threatens Superman. That anomaly, it now appears, is Doctor Manhattan. Manhattan finally comes face-to-face with Superman at the end of Doomsday Clock #11 in an encounter engineered by Veidt.
That encounter is part of Veidt's master plan to save two worlds -- Earth-0, and his own. Veidt has been holding Saturn Girl prisoner since she joined him to help locate Doctor Manhattan. In the current issue, Veidt explains his plans to her, and the importance of Superman in bringing them to fruition. Superman is also known to be plenty important to Saturn Girl, as well.
She had come back in time to stop a threat against him, after all. And she acknowledges "the future exists because of Superman" during Veidt's discussion. So the reason for her mission likely isn't just to protect Superman, per se, but to ensure the future she's from can come into existence. As Doctor Manhattan learned for himself last issue, Superman's place in reality not only impacts whether the Legion will ever exist, but whether the very future itself will ever come to be.
Saturn Girl seemingly knows this as well -- hence the whole reason for her mission. Curiously, though, as soon as Veidt asks her if the all-important Superman even remembers her -- which she admits he does not -- her very physical form begins to disappear. In moments, she's wiped from existence, with Veidt theorizing she's no longer part of the current timeline.
But why? She had existed in the current era just fine until now -- why did Saturn Girl just now fade from this reality? It's as though her realization that Superman didn't remember her is what triggered her disappearance. It's like someone stepping off the edge of a cliff and hovering in midair -- only to plunge to the ground once they remember that gravity is a thing.
In current continuity, Clark Kent was never Superboy, so he never went into the future and joined the Legion – the simplest reason why Superman wouldn't remember Saturn Girl. Superman could have met her, though, via some time-traveling adventure that brought him into the Legion's era. So the notion that he doesn't remember her at all speaks to some larger, continuity-altering reason.
She's in Another Reality Right Now -- Please Leave a Message
It's more logical to surmise the Legion simply doesn't exist in Rebirth continuity, despite Saturn Girl's presence. Doc Manhattan summed it up pretty well last issue, coming to the realization that no Justice Society in the current continuity's past indirectly led to no Legion of Super-Heroes in its future. Whatever future Saturn Girl comes from -- it's not this one, because it doesn't even exist.
Manhattan acknowledged last issue that his meddling with reality caused the intended reality to "fight back." The original Wally West's return in Rebirth was an aspect of that. Saturn Girl's arrival could be another. The Legion's future is one untouched by Manhattan, but the present most certainly has. Her presence in this continuity itself is therefore an anomaly of its own, so it stands to reason that eventually this reality -- Manhattan's reality -- would also fight back at some point.
That means Saturn Girl hasn't been wiped from all existence -- just this one. She may very well have been returned to her future, or shunted to her actual parallel past -- both of which remain untouched by Manhattan.
Her Legion ring, however, didn't disintegrate with her. This is the clearest in-continuity evidence showing that the Legion of Super-Heroes remains destined to exist.
What About Superman?
Alternatively, maybe something else transpired to cause her disappearance. When she vanishes, Superman and Doctor Manhattan's confrontation is nigh. As the possibility of a battle increases, do the odds of Superman's not surviving also increase? Is Saturn Girl's disappearance akin to Marty McFly's future family photo -- a bellwether indicative of an outcome in the past that impacts the future? Is Superman going to be killed by Manhattan, or otherwise removed from the playing field, setting the stage for the end of reality, as Manhattan has already foreseen?
Probably not. It's safe to say Superman is not going to be killed, or otherwise taken out of continuity. And the return of the Legion of Super-Heroes is already pending, so Saturn Girl isn't gone, either.
Despite Doctor Manhattan's seemingly unlimited power, Superman's recently revealed importance and connection to the "metaverse" likely comes with some power of its own. Superman's not going down without a fight – and it's a fair bet that he's not going down at all. If Manhattan is somehow destroyed, his removal from reality stands to allow it to fix itself – to stage its own rebirth, as it were. And Saturn Girl and the Legion of Super-Heroes are already known to be part of that.
No release date has been announced for Doomsday Clock #12.