Mrs Watanabe, aged somewhere between 40 and 70, nauseated by volatility, a contrarian by habit and the notional holder of Japan's household purse-strings, has some serious thinking to do.
Over the next few weeks, and with a commission-ravenous army of brokers hammering at her door, she must make a three-way call on the yen, China's economy and, with the $11.5bn flotation of Japan Post, the biggest privatisation of a Japanese state asset in more than two decades. Unfortunately, the rest of the market is not providing Mrs Watanabe much in the way of guidance.
Between now and November 4, Mrs Watanabe must decide whether she wants to buy a slice of Japan Post and the two subsidiaries, Japan Post Bank and Japan Post Insurance. These are stocks being pitched overwhelmingly to individual Japanese investors as dependable sources of yield: certainty, purr the brokers, for uncertain times.
But are the times too uncertain even for that? Much of the Tokyo market's slump since August was driven by six weeks of aggressive net selling by foreigners. The foreigner-led Tokyo market, say traders, appears to be "positioning for external calamity", mainly the fear of a China hard landing causing broad misery in emerging markets.
Those same foreigners are also being warned against getting too excited over the prospects of additional easing by the Bank of Japan later this month. "The impact that easing has on equity investments only lasts about a month and does not alter the broader trend," says Tomohiro Okawa, UBS Japan strategist, in a note to clients.
But should Mrs Watanabe trust the instincts of foreigners? Despite a recent concentration of big intraday moves, there is some semblance of calm. The is up more than 4 per cent since the start of the year, making Tokyo-listed shares the world's best-performing among developed markets in 2015. The yen is settled into an uncontroversial range around Y120 against the US dollar. Headline alarm over a Chinese hard landing has reduced from a blood-curdling roar to a low rumble.
There are even a few actively bullish signals. On a cumulative basis, corporate share buybacks in 2015 have already exceeded the record levels set last year: there is every reason, say analysts, to expect that Japanese companies will decide that the sell-off has left their own shares looking cheap, and take it as a reason to announce more buybacks along with their quarterly results in late October.
From some angles, the individual Japanese investor does look ready to turn buyer. Promisingly, the number of Japanese who have opened tax free NISA investment accounts since January 2014 has topped 9.2m, with a total asset value in those accounts of Y5.2tn. Some 50 per cent of NISA accounts held at Japan's 10 biggest brokerages have been activated for trade, up from just 20 per cent last summer. That suggests at least some of them may be readying to take on Y1m chunks of Japan Post.
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"Individual Japanese have been contrarians and turned net buyers since June. In August, investment trusts saw their sixth consecutive month of net inflows. It maybe too early to call it a full-fledged recovery by Mrs Watanabe, but we know she has got cash and seems ready to invest it," says Kathy Matsui, Japan strategist at Goldman Sachs.
But Mrs Watanabe can clearly sense fragility in her domestic market and the overhang of China continues to weigh heavily. Three months ago, the Nikkei's year-to-date gains stood at 19.5 per cent, and the yen was at an export-friendly Y125 against the dollar.
China shattered that in the space of a few terrifying trading sessions. The sudden surge of risk-off behaviour highlighted how easily the yen could revert to a strengthening phase. It is only in the past five trading sessions that the Nikkei has re-emerged from negative year-to-date territory after a sell-off in which panicking global funds used Japanese stocks as an ATM.
The underlying uncertainty remains acute. With nobody expecting a V-shaped recovery from the world's second-biggest economy, corporate Japan — and its investors — are at this point none the wiser over the force of the coming headwinds.
Kenji Abe, Japan equities strategist at Bank of America Merrill Lynch, says China represents the leading source of uncertainty for Japanese stocks, although he argues that a hard landing has already been largely priced in. Some 15 per cent of the revenues of companies listed in the benchmark Topix index are derived from China: a 1.6 percentage point decline in China's growth rate, says Mr Abe, would mean a 3.2 per cent decline in Japan's export volume to China.
Others argue, however, that China risk has not been priced in and that Japanese data have been hinting at problems for Japanese corporate earnings that will only fully emerge early next year.
"I would be surprised if China risk had been fully priced in," says Macquarie's Japan strategist Peter Eadon-Clarke. "Earlier this year, Japanese companies were guiding for profit growth of about 10 per cent this year, but then they cut summer bonuses. That must be indicating a real slowdown in their China businesses."