Lavagirl is one of the superheroes Max creates in his dreams. She is another superhero who protects Planet Drool. Her powers include general thermal manipulation, production of light, and shooting lava out of her hands. She is an ally of Sharkboy. At first she believes herself to be evil and destroys everything she touches, but she later learns that she is the light needed to defeat the darkness.
Lavagirl was at some point created and brought into reality by Max's dreams. She apparently began life in Planet Drool's volcanos. At an undisclosed point, she met with Sharkboy, and the two became allies, later forming a close (if competitive and bickering-filled) friendship. When Linus's power-lust and Mr. Electric's ambition threatened to plunge Drool into darkness, she and Sharkboy were quick to travel to earth, where they rescued their creator from a powerful electric storm. Once on Drool, she and Sharkboy assisted the seemingly powerless max in attempting to set things right on the dream-planet. The three started at Mt. Neverrest, where they briefly confronted Minus. Lavagirl defeated several plugs in battle and briefly stunned Mr. Electric, but all three were captured and dumped into the realm of Forgotten Dreams.
In the Realm of Forgotten Dreams, the trio meet Tobor, who dispenses personal wisdom to each, which briefly dispels Lavagirl's doubts about her purpose and morality. She then travels with the other two to the Train of Though aboard Tobor's disembodied facial features, where she, along with everyone else, is horrified to discover that the train is on a crash course. She is the first to bail out, and all three land in the Land of Milk And Cookies atop a massive cookie floating on.... Well, a stream of milk. Just in time to watch the Train crash. She notes the setting, then urges Max to dream, singing an off-tune lullaby and cautioning Sharkboy to treat Max more kindly, accidentally burning the marshmallow pillow Max was attempting to use to sleep in an attempt to hear the sound the other two have already heard.
Both she and her companions are once again forced to dessert (admit it, you thought of the pun too) their ride as a pair of Cookie Giants, manifestations of Max's desires for his parents to be happy, obliviously eat the cookie the three had been standing on. After briefly commenting on the situation, the group is confronted once again by Mr. Electric, who is this time supported by a legion of Plug Hounds. All three subsequently escape aboard a banana-split themed boat born of Max's imagination, journeying over the sea of confusion to the Ice Princess' castle. Lavagirl halts at the beginning of the narrow bridge of ice before the castle's entrance, fearfully asserting that her firey nature will destroy the bridge. Sharkboy proposes sleep-walking, which goes mostly successfully until Mr. Electric and his Plug Hounds once again catch up to the trio. Lavagirl is awaken by a panicked Sharkboy, and her frustration causes the bridge to collapse. All three are easily subdued and captured by Mr. Electric, who brings them before Linus, who masquerades as the deposer-turned-ruler Minus. After a brief and surprisingly genre-savvy monologue, he imprisons them below in a metal cage. Lavagirl claims her powers are weakening and fails to melt the bars. Sharkboy eventually succeeds where she failed, driven into a rage by "La Las". All three make a hasty escape, nabbing Max's Dream Journal and setting out for the Ice Castle.
Though they make good time, Lavagirl is frozen by a false Crystal Heart.
She eventually thaws, in time to remark that the genuine Crystal Heart, now in Max's possession, will not work for anyone but the Princess herself. Minus once again appears, baiting Sharkboy into a swarm of electric eels, who render him unconscious. Realizing that Sharkboy will suffocate, as his gills require forward motion to function, Lavagirl dives in after him, bringing her friend back to the surface at the cost of her own health and power. Sharkboy awakens as Minus departs, taking Lavagirl to her volcanic birthplace and immersing her in the magma there. She awakens more powerful than ever, with the epiphany that she is Light, not destructive flame, and proceeds to cleanse the Darkness from a large portion of Drool as Max confronts Minus. Later, on Earth, she saves Max's mother from the tornados born of Mr. Electric's presence.
In the epilogue, she is seen ruling over Earth's submarine volcanos alongside Sharkboy and assisting him in his search for his father. The two hold hands along a picturesque beach as the film concludes.
Some time later, a bored Lavagirl has further adventures with Max, which are chronicled in a pair of companion novels.
Lava manipulation: Being the queen of volcanoes, Lavagirl possesses the ability to control and generate most forms of lava, making her very dangerous in combat.
Light : Lavagirl discovers she is the key to defeating the darkness and lights up the sky with the power and brightness of the sun, potentially making her the most powerful being on Planet Drool.
Water: When submersed in large quantities of water, Lavagirl will go into an almost death-like state, unless she is quickly submerged in Lava.
Sharkboy and Lavagirl were holding hands when the went back to Planet Drool. Lavagirl needed Sharkboy so they went off to Planet Drool. Lavagirl kissed Sharkboy on the cheek almost at the end of the movie.