In the game, you can play as other characters. But what you are asking is something that may be implemented in the future.
If you mean Galiene then yes you can. You can go back to the ship at midnight and beat her in a fight to free her but surprisingly that doesn't move the story forward. You have to let her get possessed and wait a few days and check in at the guild hall.
(spoilers) After a few days everyone will get worried about Galiene and then the captain possessing her will return to town and let her stay as long as Galiene lets her possess her from time to time. Basically becoming like Nel and Celica.