Talk to her in the living room at night then you can buy a pair of running shoes from a vendor on a night time run, there's also a wiki you can visit that can help clear up other questions you might have.
You have to buy Yuna some running shoes. To do that, you have to go for a run in the morning and meet a shopkeeper and buy the shoes from her, and at night when you see Yuna, give her the running shoes.
