"President" John Tyler
"His Accidency", the first "usurper"?
I take it you are referring the fact that Tyler was the first VP to become President after the death of a President. There was some controversy over whether the Constitution had the Office of President, or merely had the "Powers and Duties of the said Office" devolving upon him.
Although in the absence of a provision to replace the VP, and prior to Presidential term limits, that seems something of a distinction without a difference.