Individual Details

Rachel was probably the widow, Rachel Walker, when she married Henry.
22 Oct 1796. Bond of Henry Hardin and William Comstock of Hardin County for Marriage "shortly intended to be had" between Henry Hardin and Rachel Walker.
Marriage Return of Josiah Dodge (Hardin County) lists the marriage as Oct 1st, 1796.

An SAR application from 1913 gives her name as Rachel Biddle Walker - no date of birth or death.
A FindAGrave memorial page says she died in 1850 - she is in the Census and was still living in 1855 when Henry wrote his will - so she died sometime after then.

Birth	1776	New Jersey
Marriage	1 Oct 1796	Hardin County, Kentucky - Henry Hardin
Death	Aft 1855

Spouse	Henry Hardin (1778 - 1855)
Child	Sarah D. "Sallie" Hardin (1797 - 1863)
Child	Mahuldah Hardin (1799 - )
Child	Celia Hardin (1800 - 1880)
Child	Elijah Hardin (1805 - 1877)
Child	Moses Biddle Hardin (1815 - 1885)
Child	Rachel Hardin (1818 - 1894)
Child	Nancy Hardin ( - )