Overall this was a very nice apartment in an excellent location.
Full disclosure - I am an archaeologist who has worked in Greece many years and have previously lived in Athens (including in Koukaki).
First the good:
Very nice stocked kitchen with modern fridge, stove top, and washer. You can easily cook in, etc, during your stay. The washing machine could use some instructions for guests that don't know Greek or general operations of European style washing machines.
Bathroom is renovated with an excellent shower. Much appreciated in a country where many showers still have just a hand held wand.
Lovely balcony with chairs that is great to sit on throughout the day.
Living room with two day beds - not great couches but they do.
Checkin and getting keys was very easy - no first going to some office on the other side of Athens!
Now a few critiques:
The door to the apartment is very old and lets a significant amount of sound through. It's very easy to hear all the lobby comings and goings. Replacing it with a new model would greatly improve the situation.
Associated with this is the unit's location directly above the building entrance. The door entrance buzz and slaming is heard and felt through the floor easily. Mostly a thing during the day - things do quiet down at night.
The bed frame was quite squeaky. I move a lot in my sleep, and with every roll the frame would shake and squeak.