The check-in didn't go very well. I didn't have a sim card while in Europe so I tried to get all the details while I had wifi at the airport. While Charlie told me the code to the gate and where I could find the keys, he did not mention that there are more than one building in the court yard or which apartment I should be looking for. I ended up moving around with a heavy suitcase in the rain, trying to figure things out for almost an hour until a super nice girl let me into her apartment to use internet and she called Charlie to explain the situation. I do want to mention that Charlie did wait for me in the apartment but my flight was delayed, so he had to leave before I arrived. However, given the lack of information, him being in the apartment might not have solved the problem either.
The wifi only worked very briefly when I arrived. When I realized that the wifi wasn't working, I had the problem of getting into touch with him again. I had to bother the aforementioned girl because she was nice enough to leave all the information for me to find her. It took a little bit time to get in touch with Charlie. The girl called him. He said he was on his way and would call back when he arrives but didn't. I waited in the girl's place (feeling uncomfortably like an inconvenience) and finally decided to go back to the apartment to check whether Charlie had arrived, and turned out he was there already. Unexpected internet breakage is totally normal and it is never easy to get the cable company to come fix it, let alone it was a Saturday evening with an upcoming public holiday on Monday. Charlie understood that it was essential for me to have internet access during my stay, so he let me use his spare sim card. It was very generous of him and made a lot of things easier for me. I have no complaint there, except the first afternoon/evening of dealing with check-in and wifi issues was exhausting.
I had no other issues with the apartment for the remainder of my stay. The place is nice and clean. Location is very convenient, close to metro, Gare du nord, sacre coeur, and all the entertainment in the district. It is a busy street outside the windows but once windows are shut, there was no noise issues and I slept very well. Charlie seems like a very nice guy, and there was no communication issues later either.