The following from the Pendleton Dist SC Deeds 1790-1806 help in idenfying who Mary Ellen Powell's father was:

#381,382
James Powell(called Sr. in Oath) and Ales Powell, his wife, for love and affection they bear to beloved children, Sam Stevens and Polly{Mary} Stevens, his wife, grant all household goods and property now in our possession, also all claims and demands we have against any person. Date: 19 Sept 1805 signed James(x)Powell and Ales(x)Powell. witness: David Murphree, Allen Powell. Alllen Powell made oath to David Murphree, J.P. 26 Oct 1805

This undoubtedly refering to the older James Powell age 45up in the 1800 Pendleton census. Sounds like sort of a will or perhaps planning to move? This James is not in the 1810 Pendleton census. The James in the 1810 census is the James Powell who married Rebecca Bynum, living next to Samuel Stephens. James wife is Ales(Alice?). One can speculate that she was Alice Brewer, a daughter of one of the Brewers who lived earlier in Chatham/Orange counties NC and one finds a few land transactions there between Brewer and a James Powell. This James Powell probably one of those listed in the 1790 Chatham census.

Who was the Allen Powell who witnessed?
Also in deed book:

James Powell, Sr., for confidience in my trusty friend, William Powell, appoint him my attorney to recover all debts and demands due me in Chatham or Wake Co NC Date: 21 Jul 1802. signed James Powell. wit: Samuel(x)Stephens, William Murphree, James(x)Powell.

Here we have James sr being witnessed by his son-in-law Samuel Stephens, by William Murphreeand a James Powell; perhaps this is James Powell who married Rebecca Bynum. The William Powell either his brother or son. James has ties back to Chatham NC.
Samuel Stephens and Mary Ellen Powell Stephens named their sons James, Allen, William, Greenberry and Dison. My ancestor James Dison Powell b.1824 SC. migrated along with Allen, James, William and Josias Powell from the Pendleton, Pickens Co., of SC before 1850.