Booking Kofumi's place is the most right decision, it is convenient (4 min walk to Okubo station, 7 min walk to Shin-Okubo station), yet quiet in day and night (besides some passing by people in day time). It has several convenient shops (7-11, and Family Mart) and many eating place nearby, along the road to the metro stations.
The house is ultra clean, though it is small, it has everything we need. And the facilities are in great condition, I even used the bath-tub because it is so clean (which usually I will not choose to soak in). The toilet bowl is better than the rest we used, and the bed, we really enjoyed our sleeps there. Very comfortable mattress and pillows and quilt, feels like home. Aircon works well for both warm and cold. Also, enough storage for hanging your cloth. There is even a curler for girls.
Kofumi is an awesome host, though we never met her, she made sure everything is ok during our stay, and even remind us of wet weather and prepared umbrella for us. We arrived at 1am in the first night without being able to be reached, and her husband had been waiting for us all the way by the road side, we were so touched. It's a perfect place for first time visitors in Tokyo who prefers convenience and a comfortable stay. Thank you Kofumi san!