1. "I reserved this specific area just for us, hahah."
2. She's naked!
3. Onsens, or communal hot springs, adhere to ancient traditions which include bathing naked.
4. The majority of onsen doesn't allow you to wear bathers or any other clothes while soaking in the hot spring. That's why kir is naked.
5. However, there are a few hot springs, mostly natural outdoor and mixed-gender onsen baths, where women can wear a towel.