Who was George Washington? Why was he good at bipartisanship? How did he warn citizens after he left office?

George Washington served as general of the Continental Army during the American Revolution and also served as the nation's first president between 1789 and 1797. It's important to remember that when the Continental Army defeated the British to win American independence, America was not a nation in almost any sense of the word. It was a loose confederation of 4 million people widely spread people in disparate communities with little in common and deep regional differences. The citizens of the new America really only agreed on one thing: George Washington. Without him, it’s likely the new states would never have successfully come together or stayed together.
There were two distinct factions within Washington's Cabinet—the Democratic-Republicans, led by Thomas Jefferson and James Madison, and the Federalists, led Alexander Hamilton and John Adams. For much of Washington's two terms in office, Washington mediated between these two factions, hearing both sides and then acting decisively. Due to his personal popularity, he was largely successful until, in Washington's second term, the rift between the two sides, fueled by a vicious, scandalmongering press, became so wide that Jefferson resigned his post as secretary of state.
When Washington left office, he feared that the divisiveness of a two-party system would destroy the country. In his famed Farewell Address, published at the end of his second term, he warned his fellow citizens about three specific dangers he saw to the future of the United States—partisanship, debt, and foreign wars. In regards to parties, he wrote that although their existence was a part of human nature and they might, in moderation, provide a useful check on executive power, they also could be a "fatal tendency" and that when led by "cunning, ambitious and unprincipled men," they could be used to undermine democracy and the good of the whole for the gain of a few. He went on to warn that parties could be used to manufacture differences between people of different regions, which could then “render alien to each other those who ought to be bound together by fraternal affection.” Washington cautioned that, ultimately, parties could lead to the rise of despots and the death of democracy. 
https://www.ourdocuments.gov/doc.php?flash=false&doc=15&page=transcript