Short answers, pulled from the story.
Helmut Kohl served as chancellor of Germany from 1982 to 1998, a period of sixteen years. This is the longest chancellorship in German post-war history and the longest tenure of any democratically elected chancellor of Germany.
Kohl was the principal architect of German reunification. He presented a ten-point plan for overcoming Germany's division, secured assurances from Soviet leader Mikhail Gorbachev, and signed the reunification treaty on the 31st of August 1990. East Germany officially ceased to exist at midnight on the 3rd of October 1990.
The Maastricht Treaty established the European Union and the Euro currency. Kohl and French president Francois Mitterrand were its chief architects, building on a close Franco-German partnership that began visibly with their joint commemoration at Verdun on the 22nd of September 1984.
Kohl became chancellor through a constructive vote of no confidence on the 1st of October 1982, after the SPD/FDP coalition collapsed over economic policy. It remains the only time in German history that a chancellor has been removed by this mechanism.
The scandal became public in 1999 when it emerged that the CDU had received and kept illegal donations during Kohl's leadership. Der Spiegel reported that Kohl had led the party's financial system outside legal boundaries through secret bank accounts and civic associations used as intermediaries, though it was not suggested that he personally benefited from the donations. Kohl resigned as CDU honorary chairman in 2000 as a result.
Kohl received the Charlemagne Prize in 1988 jointly with Francois Mitterrand, the Prince of Asturias Award in International Cooperation in 1996, the Presidential Medal of Freedom from President Bill Clinton in 1999, and was named Honorary Citizen of Europe by the European Council in 1998. The latter honor had previously been given only to Jean Monnet. After his death, he was honored with the first-ever European act of state, held in Strasbourg on the 1st of July 2017.