Tyler's doctor: I hope not, sir.
John Tyler: Perhaps it is best.
John Tyler (March 29, 1790 — January 18, 1862) was the tenth President of the United States, the second from the Whig Party, and the first Vice President to inherit the Presidency. He served from 1841 to 1845, preceding James K. Polk and succeeded William Henry Harrison after the latter died only a month into his presidency.
Upon succeeding office, Tyler was nicknamed "His Accidency" by his detractors. He was a longtime Democratic-Republican who was elected to the Vice Presidency on the Whig ticket, so he was rather unpopular and had several disagreements with his former political supporters and had a tenuous relationship with his predecessor during his short Vice Presidency. Tyler declared that he was now the President rather than the Acting President. He vetoed several of the bills brought to the White House and, as a result, most of his Cabinet resigned and Tyler was eventually expelled from the party because he couldn't cooperate with them. In fact, the first presidential veto to be overriden by Congress was one of his. He's also the first president to have an impeachment attempt directed against him — on the somewhat thin reasoning that actually using his veto counted as an abuse of power, even though nearly every previous president had vetoed at least one bill, and Andrew Jackson had vetoed more by the same point of his presidency — but the initial vote failed 83-127, so that's probably why you never heard of it. However, he was responsible for the annexation of Texas, which is something worthwhile. He signed the Webster-Ashburton Treaty with the United Kingdom (which resolved border issues between the U.S. and British Canada in the northeast, between Maine and New Brunswick), the Treaty of Wanghia with China (which gave America the same trade rights that the European powers were starting to get in China), and added Florida to the Union on the last full day of his presidency. After the South seceded, Tyler was elected to the Confederate House of Representatives but died before taking office. He was buried with his coffin draped in the Confederate flag, making him the only president buried under a "foreign" flag, and the only one whose death was not officially recognised in Washington.
He fathered eight children with his first wife, who died while he was in office, and remarried while he was president, making him the first President to get married during his Presidency, fathering seven more children with his second wife (whom he married after her father was killed in front of both of them by an exploding experimental cannon and she fainted into his arms). The second Mrs. Tyler asked to be addressed as "the Presidentress" which didn't catch on. In addition to those fifteen legitimate children, there are rumors of him having an illegitimate child. According to The Other Wiki, two of his grandsons are still alive (born 1924 and 1928 respectively - their father, Lyon Gardiner Tyler, was born in 1853 when Tyler was 63 years old meaning that the younger of the two grandsons was born when Lyon was 74 or 75 years old).
Rasmussen Reports, an American polling agency, polled citizens on what they thought of each President in 2007, and 76% could not give an opinion on him. If this is any evidence, he's probably the most forgotten American President. He is not helped in this sense by having a relatively bland name, unlike, for example, Millard Fillmore.