I love this question so much but it is impossible to answer. I have formed so many attachments to fictional characters that at this point to only bring one to life would feel like I was betraying all of the others.
Assuming that I would not be visiting any fictional worlds but rather staying here on Earth, I think that if I HAD to choose one character to spend the day with, it would be Beth March.
Beth March (from the coming of age novel 'Little Women) is, in my opinion, the most overlooked of all four sisters in the book. She is the most reserved and shy of the four, but she is consistently sweet and selfless. I would want to spend the day with her because I feel like i made a strong connection with her, from the point of view as a reader. Her strong moral compass and kind-heartedness would just be so uplifting to witness, and I just KNOW we would have the best time playing with my neighbours cats.