Lei is quick to respond to messages. His place is located what seemed to be less than 100m off the main street from all the restaurants and stores of Koreatown. This also means that if you're a sensitive sleeper, you may hear the sirens of ambulances that use that road in the latter hours of the evening. There also seemed to be a possible stray cat in the nearby area that kept going to the neighbours unit and meowing all night. My family saw it, and it didn't seem to have any identification tags or collar.
Also note that the Shin-Okubo (off the JR Yamanote line) station does not have an elevator or escalator, and only has one entrance leading up to the platform. If you have large luggage that you cannot easily lift, you may want to use the Okubo station off the JR Chuo-Sobu line instead, and wheel everything an extra distance.
Inside the house, it's quite comfortable. Large and spacious rooms. The staircase is a little narrow, which is traditionally Japanese in width and steepness. Would not recommend bringing large luggage up them unless you're careful. I noticed that previous guests have scuffed the stairwell walls and chipped the edges of the stairs.
A lot of the windows have no view from them (due to population density), but can offer an energy free way of cooling the house in the evening, after a long hot day. If it's not cool in the evenings, each bedroom has their own heating/cooling unit. So everyone can still sleep comfortable. The bedrooms are quite spacious, and Lei's place can easily hold 4 people on the third floor and 4 on the first floor. Just note there's only 1 shower for that many people.
Overall a good place to stay.