You're stuck in a white room.

This is for scientific purposes.

For 100 days, you are stuck in a plain, white room, with two beds, one functional bathroom, and a regular supply of food delivered with a complex system, along with some stationary and a notepad. There will be lights on at all times. You have no contact with the outside world save for those keeping you there, and you can only bring one specific thing in (e.g. a book or a board game). Thankfully, you can also bring in one character from Project Sekai to keep you company. If you both survive and sustain your sanity, you will win $500,000.

What item will you bring? And which character would you most like to suffer with you?