Seiji Watanabe's enchantment with the dynamism of stock trading helped him rise to a senior position at Nomura Securities, Japan's biggest securities house, overseeing sales of Japanese equities to institutional investors.

His deep knowledge of Japanese stocks even made him a go-to person for many money managers worldwide seeking advice on investing in companies such as Toyota and SoftBank Group.

In his late 40s, however, Watanabe turned to a harder sell — trying to convince a Japanese public known for its relative lack of enthusiasm for investing that stock trading can be a worthwhile endeavor, offering not just personal financial rewards but, in his view, benefits for society, too.