Exposition Lass (Pages 4-5, 10, 26)
While we're still a little leery of Saturn Girl – if that is her real codename – we must admit that she acts pretty benign in this issue. Although Ozymandias calls her a "bubbly blonde" (Page 4), honestly it's nice to have someone who's optimistic about how the story ends. That makes her look of horror on Page 26 even more effective, and it also helps sell the occasional bit of backstory.
We do have a quibble with her introduction to Ozymandias on Page 4. For decades DC (and Marvel) superhero comics have always taken place in "the present," creating an elastic timeline where events aren't usually pegged to particular dates. Nevertheless, traditionally the Legion of Super-Heroes' adventures take place 1,000 years in the future. Therefore, when Saturn Girl says that she's from the 30th Century, depending on her age it could be the 31st Century.
Saturn Girl's explanation that "Superman's why [she] serve[s]" (Page 5) goes to the Legion's underlying rationale. As explained in April 1958's Adventure Comics issue #247, the Legion was founded after the example of Superboy, the 20th Century teenager who (at that time) grew up to be Superman, the universe's greatest hero. The 4-issue 1984-85 miniseries Superman: The Secret Years (written by Bob Rozakis, pencilled by Curt Swan and inked by Kurt Schaffenberger) depicted the transition's specific events. Regardless, John Byrne's 1986 Man of Steel miniseries eliminated the Superboy career, establishing that Clark Kent adopted his Superman identity as a 25-year-old. Likewise, the Legion's history was revised a few times, but still tended to take Superman as its inspiration. In recent years, stories like Geoff Johns and Gary Frank's Superman: Secret Origin issue #2 (December 2009) have introduced the notion of the Legion visiting teenaged Clark Kent and bringing him forward in time to have Superboy-costumed adventures with them.
As one might have expected, Saturn Girl's mission is to "cleanse the time stream of an unknown anomaly that threatens [Superman]." This is nothing new. As recently as the New 52, the three founding Legionnaires came back in time to save Superman in Action Comics issues #6 (April 2012) and #17-18 (April-May 2013).
Saturn Girl's Page 10 remark that Johnny's reunion with the Thunderbolt is "history" is as good a time as any to mention that three different Legions of Super-Heroes, with three different Saturn Girls, have interacted with DC-Earth's present-day super-people. Based on this one's demeanor and her Superman comments, this Saturn Girl belongs to the Legion from this Earth's future. The other two, representing the reboots of 1994 and 2004, are from the futures of Earth-247 (a world destroyed in Infinite Crisis) and Earth-Prime (a world with no present-day superheroes).