Changes in Yukawa's life in suburban Tokyo had been fast and disorienting. Over the past decade, he had lost his wife to lung cancer, lost a business and his house to bankruptcy and been forced to live in a public park for almost a month, according to Yukawa's father and an online journal he maintained.

The hard times led to soul searching. By his own account, he had changed his name to the feminine-sounding Haruna, attempted to kill himself by cutting off his genitals and came to believe he was the reincarnation of a cross-dressing Manchu princess who had spied for Japan in World War Two.

By late 2013, Yukawa had also begun a flirtation with Japan's extreme right-wing politics and cultivated a new persona as a self-styled security consultant, according to his Facebook page and blog posts, though he never did any work as a consultant.

The Japanese hostage crisis appears certain to play a role in the debate over Japan’s global security posture as Prime Minister Shinzo Abe ’s government prepares legislation that would give it more freedom to use military force.

Mr. Abe’s government on Monday convened a session of parliament in which it plans to introduce legislation that will allow Japan to engage in “collective self-defense,” including aiding allies such as the U.S. in regional conflicts threatening Japan’s security, and to come to the rescue of Japanese citizens abroad.

Koichi Nakano, a political-science professor at Sophia University in Tokyo, said the hostage crisis—in which a Japanese citizen was beheaded by Islamic State militants last week—could strengthen Mr. Abe’s resolve to restyle Japan as a more muscular political actor on the international stage.

“Mr. Abe may use the latest case to raise support for his move to expand the role of Japan’s military,” he said.