The Betrayal of the Guardians by Diana Calanguia

When Arisa joined the trials to become a new Astral Guardian, she never thought she would have to fight for her survival. Arisa, Kazuya, and Zen have been friends since childhood. For Arisa, their friendship is what makes her smile and what made her develop deep feelings she had to forget. Living in a world that suffers attacks from other worlds' creatures, the chosen Guardians once fought for her world's survival. Blessed with the powers given by the Stars, those same Guardians have now become threats themselves. Despite hearing tales of when Guardians were still heroes, Arisa refuses to grow a liking for them. Even when one is always there to save her from troubles, she doesn't believe that they will ever return to the heroes they once were. Due to their more recent despicable behavior, she wishes she could do something to stop them. Her wish is granted when she is chosen as a candidate to compete for a spot to become a new Guardian. Even though she hates the Guardians, she has to become one of them if she wants to overthrow them. She accepts the challenge. But she never knew that the road to becoming a Guardian would be more challenging than she ever imagined.