The more good things you do with her, like bathing together and praising her will make her like you more and increase the time she travels with you, at some point she will like you so much that she never leaves your team
You need to finish to the final Cocona quest, fight Big Mama to let her agree that you can leave Noer island with Cocona. Once you have finish that part and get Big Mama approval, you will get nil duration for hiring Cocona and she will never leave until you ask her.
