Koichi's apartment is fairly conveniently located in a quaint residential neighbourhood area just off the eastern edge of the hustle and bustle of Shinjuku. A short 10-15 minute walk will get you to a number of local eateries and Shinjuku Sanchome subway station (which affords transport access to most parts of Tokyo). A 30-40 walk will get you to most of the rest of Shinjuku.
The apartment itself is clean, tidy, quiet and comfortable enough for 3-4 adults
(and a child or two, camped out on the living room divan). Don't be too put off by the presentation of the apartment building, which appears a bit tired in its exterior and common areas.
There is an excellent patisserie/French-style backery beneath the apartment (which, sadly, only opens from about 11.00am, and runs out of stock by late afternoon). There is also a laundromat, a Dominoes pizza outlet and 3-4 local convenience stores closely located to the apartment and from which pork buns, fresh fruit, drinks and other fresh packaged pasteries are available for breakfast fare.
Make maximun use of Google maps with your own mobile data plan, or Koichi's supplied wifi modem; and a downloadable Tokyo subway/railway map; and you should have no trouble finding you way around Tokyo. If more local maps are needed, pick up some Shinjuku tourist information, or some local maps from nearby hotels (if Koichi doesn't already make them available to you).
Koichi himself is a friendly and most accommodating host with an office located very near to his apartment.