Who did PercyJackson fail to save?

Percy Jackson failed to save his friend and fellow demigod, Luke Castellan, during the events of "The Last Olympian." Despite their complicated relationship and Percy’s efforts to reach out to him, Luke ultimately succumbed to the influence of Kronos and chose his path, leading to his demise in the battle against the Titans. Percy's inability to save Luke highlights the themes of friendship, loss, and the struggle between good and evil throughout the series.