Everything was perfect. Christina has a beautiful appartment, a little outside of the center of Porto in a quieter neighborhood. To and from the center it is approxximately 20-30 minutes walking; alternatively you can use the bus, metro (~1-2€ for a ride) or Uber (~3-5€ for a ride from the city center, if I remember correctly).
The appartment has a separate bathroom for guests, with its own toilet and bathtub/shower. The spacious guestroom features a small couch and a large double bed. The kitchen, which is shared with Christina, seemed well equipped with everything needed to prepare your own meals, if you choose to do so.
In case you want to take a break from exploring the city and just relax a bit, there is also a very nice rooftop terrace.
Christina responded quickly to any questions per Whatsapp or on AirBnB and took a lot of time to give us advice on how to experience Porto, when we arrived. She also gave us a city map, marked with all the spots she had recommended to us. Last but not least, after our stay she even sent us some jewellery to Germany, that we had forgotten at her place.
All in all, we can definitely recommend Christina as a host and would happily book her place again.