When Galactus needs sustenance, he sends his herald ahead of him to find a planet suitable for him to consume. The Silver Surfer, aka Norrin Radd, may be the most popular herald to the Devourer of Worlds, but he was merely the first. While Radd spends his likely century-long service to the galactic glutton trying to find worlds devoid of life, so that no innocents are killed, Nova — whose real name is Frankie Raye — isn't as picky.
Frankie Raye is doused with a chemical that gives her superhuman powers similar to those of the Human Torch when she is a child. After a romantic fling with her fellow flame-engulfed hero, she volunteers to enter into the service of Galactus and find planets for him to consume. The Fantastic Four are understandably concerned about her willingness to wipe out countless lives, while Galactus is more than happy to put it to good use.
One thing to keep in mind is that while Nova would be a formidable adversary to the Fantastic Four, she would also be the perfect villain to match Carol Danvers, aka Captain Marvel (Brie Larson), punch for punch in a space battle. While the Fantastic Four deals with Galactus, Captain Marvel could try to help Frankie see the error of her ways — if she shows up to help out at all.
