Phillis was enslaved by Waldo Emerson, and likely passed on to his wife and then their daughter.

"In all inventories they [enslaved people] were generally classed with the stock on the farm, or with the animals of the homestead. So also in all wills. In the inventory of the estate of Waldo Emerson, who lived where Henry Kingsbury now does, is the following: '1 negro wench named Phillis £30.0.0, 1 large horse £6.0.0, 1 Mare £18.0.0.' - Bourne, p. 408

Bourne's quote is not quite accurate. The actual document states "1 Negro Wench Called Phillis £30.0.0, 1 Large Horse £6.0.0, 1 Mare £8.0.0," not £18.0.0. This entry is near the bottom of page 19 of the 23 pages in the estate inventory.

Note: "Negro wench" was used most consistently to describe a female slave of at least child-bearing age. FMI: https://onthemarks.net/2018/07/12/negro-wench-appendix/

According to Bourne (pp. 492-93), Waldo Emerson "had three children: Samuel born April 25, 1760, Sarah born May 18, 1762 and Waldo born March 20, 1764. Samuel and Waldo died in infancy. His widow survived him but a short time, dying the 23rd of June following, at the age of thirty-two, leaving thus only daughter Sarah, who was married to Theodore Lyman Nov 21, 1776." His estate was presumably left to his wife, then passed to their surviving daughter Sarah, and potentially to her husband Theodore Lyman.

Bibliography:

The History of Wells and Kennebunk from the Earliest Settlement to the Year 1820 - by Edward Bourne (1875)

York County Registry of Probate

Maine Probate Abstracts Vol I 1687-1775 - John Eldridge Frost (1991)