I highly recommend Kentaro's place. The apartment is in a great neighborhood. It's just locals and no tourists. Very quiet, clean and with everything you need for a short or long staying.
The apartment is easy to get to from Ueno station (15 mins walking). It's very simple but with everything you need and more.
Kentaro had even toothbrushes for us. Those turned out to be very handy when we discovered that our luggage didn't make it and was going to arrive the next night. Actually Kentaro was incredibly nice and agreeing to receive the luggage for and drop it off at the apartment. Because without speaking Japanese it was going to be very hard for us to coordinate the delivery. Thank you again for that Kentaro that really saved our trip.
As an extra Kentaro speaks fluent Italian and we exchange all our communications in Italian.