Stay at Kai's place was very nice and comfortable. The area is beautiful and peaceful and even if I didn't have a key of the room I was feeling comfortable and safe. The bed was very great and I could also do the exercises in the room as it is a big space. I had a great sleep as it is really quite area. When you open the window you are under the ground but you see the garden which is green and nice. I also met nice people there, the kitchen is nicely equipped, you have your place for everything, the space is really large so you have all the comfort you need. Kai was very welcoming and careful and thank you for that!
Also, someone wrote in the previous comments that the washing machine next to your apartment is loud - not at all and there were washings very rare, I didn't hear it at all. The internet was great what is rare with the Wi-fi in Berlin.