Short answers, pulled from the story.
Joachim von Ribbentrop and Kintomo Mushanokōji signed the Anti-Comintern Pact on the 25th of November 1936. They acted as representatives for Nazi Germany and Imperial Japan respectively.
The secret additional protocol explicitly targeted the Soviet Union by name rather than just communist activities generally. It obligated both signatories to consult each other before engaging in any bilateral treaties with Moscow.
Hiroshi Oshima drove negotiations between Berlin and Tokyo while operating outside official diplomatic channels. He utilized his fluency in German and connections with figures like Wilhelm Canaris to advance pro-German agendas within his embassy.
Italy joined the Anti-Comintern Pact on the 6th of November 1937 following the failure of the Stresa Front initiative. Mussolini sought new partners after League of Nations sanctions isolated Rome due to its invasion of Ethiopia earlier that year.
The Anti-Comintern Pact transitioned into the September 1940 Tripartite Pact which identified the United States as the primary threat instead of the Soviet Union. This shift rendered the original agreement virtually inoperative by December 1941 when war broke out between Japan and America.