Answer:
Nico made Percy promise to use Annabeth's invisibility cap to protect Bianca while on the rescue mission to get Annabeth back.
Step-by-step explanation:
"The Titan's Curse" follows the story of Perseus and his quest to defend the world and his life from evil forces. This book is the third series from "Percy Jackson and the Olympians" written by Rick Riordan.
In this story, an evil monster had captured Annabeth and a rescue plan was made. And for this rescue mission, the Hunters of Artemis along with Bianca are chosen, leaving Nico, Bianca's brother, and Percy behind. Nico and Bianca are the demi-god/ half-blood children of Hades.
Percy had been eavesdropping on the conversation between the Hunters and Bianca about the rescue plan. And when Nico found out that Percy can become invisible with Annabeth's invisibility cap, he requests him to follow the rescue team and make sure to protect his (Nico's) sister Bianca.