Short answers, pulled from the story.
Dmitry Donskoy was Prince of Moscow from 1359 and Grand Prince of Vladimir from 1363 until his death on the 19th of May 1389. He is famous for leading the first Russian military victory over the Mongols at the Battle of Kulikovo in 1380, fought beside the Don River, from which his surname derives. He is venerated as a saint in the Russian Orthodox Church and regarded as a national hero of the Russian Middle Ages.
At the Battle of Kulikovo in 1380, Dmitry Donskoy's forces defeated the Mongol army led personally by Mamai, a general and claimant to the Horde throne. The battle opened with single combat between two champions: Alexander Peresvet, a warrior monk sent by Sergius of Radonezh, and a Horde warrior named Temir-murza. Both men killed each other on the first pass, and the broader battle ended in a Russian victory.
Sergius of Radonezh blessed Dmitry Donskoy before the Battle of Kulikovo, but only after satisfying himself that Dmitry had pursued all peaceful means of resolving the conflict. Sergius also sent two warrior monks, Alexander Peresvet and Rodion Oslyabya, to fight alongside the Russian forces.
Dmitry Donskoy was canonized on the 6th of June 1988 at the Trinity Lavra of St. Sergius by the Local Council of the Russian Orthodox Church under Patriarch Pimen I of Moscow. His feast day is the 19th of May, the anniversary of his death.
When Dmitry died in 1389, he became the first Grand Duke to bequeath his titles directly to his son Vasily I of Moscow without seeking the Khan's approval. This was unprecedented and represented a break from the Mongol practice of controlling succession among Russian princes.
By the end of his reign, Dmitry Donskoy had more than doubled the territory of the Principality of Moscow. He built the stone Moscow Kremlin, completed in 1367, which helped the city withstand two sieges by Algirdas of Lithuania. He also secured the title of Grand Prince of Vladimir and won recognition of his authority from other northeastern Rus princes.