In many ways, this was a war brought on more by historical forces than by individuals. However, if we are going to try to identify individuals, I would point to two men.
The first is William Randolph Hearst. He was a newspaper publisher who was strongly in favor of the war. He believed that publicizing Spanish atrocities against Cubans and pushing for war was a good way to sell newspapers so he pushed hard with his "yellow journalism" for war.
The second is Enrique Dupuy de Lôme. He was the Spanish ambassador to the US. He sent a letter in which he made very critical remarks about President McKinley. This was seen as a major insult to America and helped bring about the war.