Daughter of Neptune (Heroes of Olympus)

After her mother's death, Marina Garcia had became an adventuress with her equally adventurous partner and mother figure, a immortal lioness named Mambi. She was told that she's a daughter of Neptune and she had finally arrived to Camp Jupiter for a home after navigating the labyrinth. But one day, when she was touring the labyrinth, she had came across a unique group of people who calls themselves the Heroes of Old.
She learns about the Sea God's Prophecy, she learns how she and Percy are both involved in the prophecy. But there'll be a great deal of challenges for them to face and have to depend on each other to survive.

(Don't worry, I'm writing for fun).