Elpis

 Elpis (Ancient Greek: ἐλπίς) is the personification of hope, perhaps a child of Nyx and mother of Pheme, the goddess of fame, renown and rumor. She was depicted as a young woman, usually carrying flowers or cornucopia in her hands. Her Roman equivalent was Spes.

History
In Greek mythology, Elpis was both the goddess and spirit of hope. She was the only good placed inside the box which was given to Epimetheus and his wife, Pandora. When Pandora released all the miseries in the jar, she managed to close it shut before hope could escape, preserving her in spite of the evils that had been unleashed on mankind.

The Last Olympian
When Prometheus was enlisted to organize a truce between the demigods and titans for a peaceful end to the fighting, he had Pandora's box in his possession. When Percy Jackson refused, Prometheus gave him the jar so that when he was ready to surrender, all he had to do was release her, symbolically "giving up hope." Throughout the entire battle (though Percy was tempted to), Elpis was never released. In the end, Percy gave her to Hestia for safekeeping, because "hope survives best at the hearth." Hestia has since retained guardianship of the pithos.