Forgot to pack underwear! Go to Uniqlo!
Any season, the place to go is Uniqlo - they have sizes for just about everyone now (didn't use to), and their stuff is basic, well made, reasonably priced, and simple enough to go with most peoples' styles. I have taken people there several times when they've arrived in Japan without a key piece of clothing due to weather change or whatever.
Uniqlo is where you get your basic stuff, and if you want to get a little funky, there are plenty of other places in Tokyo for that too!