The Principle Guardians are the most powerful and oldest of all the Guardians. Their blood is very powerful and is very delightful and delicious to vampires. Mixed with human blood, a Principle Guardian’s blood is extremely fatal to the Old Ones if drunk by them. Elena is an example of Principle Guardian’s blood mixed with human blood as she is a Principle Guardian-Human hybrid, her blood is the only thing that could kill the Old Ones and prevent their revival.
In the novel, Elena discovers that she had Guardian Powers and that they weren't there by chance: her mother and father were part of a society dedicated to light magic and a Guardian came to earth saying that he was looking for two parents who would be the parents to a Guardian who would the protector of the world and who the Guardians would take when she turned twelve. Guardians then killed her parents when they refused to give her away because they’d bonded with her. Elena is heartbroken by this that her parents would want to give her away to those cold women who killed her parents in the first place when she turned twelve. She wonders if they still want to recruit her.
In the novel, Elena Gilbert discovers that she is an Earthly Guardian and that there’s another Earthly Guardian out there. She wonders if Earthly Guardians are different from Celestial Guardians and if they’re more humane. Elena’s teacher calls this new Guardian called Andrés from Costa Rica to meet Elena. After meeting him, Elena sees a very human guy and she eases on the idea of being a Guardian herself as Andrés teaches her to control her aura and see and track the auras of others. Andrés tells her that when the Guardians give her task, more of her Powers will be active. A Principle Guardian called Mylea comes to Earth and officially makes Elena a Guardian with a pledge and the symbol of the Celestial Court appears on her palm: ∞. But Mylea gives her a task that she doesn't want by telling her that she had to kill Damon in order to access more of her powers, should she fail, she will be reassigned and will be sent to be a member of the Celestial Court forever. Mylea leaves before Elena can say otherwise. Elena later fights against Klaus using her awakened Powers and Pyrokinesis and succeeds in stopping him from harming her friends further, but Klaus manages to take her before the house burns down with them inside and holds her hostage at the tunnels leading to the hideout of the former Vitale Society. Klaus bites Elena claiming that he’d never tasted a Guardian before. Stefan and the rest of Elena’s friends arrive and Klaus grins with Elena’s blood around his mouth. He then shudders and falls to the earth motionless and Stefan has no doubt that he is dead. Andrés comments that Elena is the “One” and that her blood could kill the most ancient power Old Ones; the First most evil Demons, Vampires, Werewolves, Witches and Monsters who’ve existed since and before the dawn of human civilizations and Elena is the child of a Principle Guardian. Bonnie asks if Elena’s parents were Guardians, but Elena needs to recover. Later, Andrés comes to Elena’s room and gives her a book and tells her that it may help her. The book is a journal and it once belonged to Elena’s mother. In the diary, we learn that Elizabeth (Elena’s mother) was once a powerful Principle Guardian who was assigned by her sisters to mate with a human and have a child with both human and Guardian blood that could kill the Old Ones forever. It is revealed that Elizabeth had given birth to Katherine von Swartzschild and that Katherine was abandoned by the Guardians because she’d had a sickly life, so her sisters gave her another assignment once she’d given up Guardian life. Elizabeth refused, but her sisters the Guardians, begged her and she said yes on the condition that her daughters be given the choice once they were old enough to decide for themselves. The Guardians didn't want to wait any longer, but they broke their deal and sought to take Elena earlier when she turned twelve. Elizabeth and Thomas paid with their lives and sadly died. Elizabeth writs that her daughters will one day claim incredible powers and wishes for Katherine, Elena and Margaret (Elena’s little sister) to one day reunite. Elena is relieved that her parents didn't want to give her away, but they would've waited until they were old enough.