Megan is a special character in the book; after all, her nickname is the namesake of the novel: Firefight. The nickname points to her supernatural abilities. She fights fire with fire, so to speak, because in the battle against supernatural Epics, she is able to use her own abilities to sway the fight in the way of the good guys. That doesn't make her automatically good, as Mizzy points out, but when she sacrifices herself for the team, her motives are shown to be truly good.
One interesting question is the playful (yet deadly) scene where David tries to kiss Megan and she points a gun at him. The safety is on, which he takes as proof of love, but the symbolism is dense and confusing. The gun is a tool of death, but she is only pretending to threaten him, it seems. Later, they kiss, and there is no gun. Perhaps this is a symbolic way of describing what it feels like to be in love, as if someone holds the button that could kill you.
But is that kind of love sustainable? Perhaps the answer comes with a deeper understanding of what Megan represents. David is a water warrior by the end of the novel, and his transformation from drowning, helpless boy to master of water is a growing process, but Megan is automatically magical. Her journey brings her to her demise, and she isn't part of the group in the same ways that other characters are. Perhaps Megan's character is designed to reveal part of David's character. She might represent his full potential in life, but packed into an external character whom he adores.
