Mizuho Financial Group Inc. said Friday a subsidiary has agreed to acquire an approximate 20 percent stake in online brokerage Rakuten Securities Inc. for approximately 80 billion yen ($554 million). Mizuho Securities Co. will acquire the shares on Nov. 1, as the financial group attempts to expand its reach among younger customers, who make up the main clientele of Rakuten Securities. Through the new partnership, Rakuten Securities, which is owned by Rakuten Group Inc., will be able to introduce face-to-face services offered by Mizuho, as well as allow its customers to trade a wide range of pr…