Fidelia Leybold and Frederika Schmidt

Detail showing "Fidelia Leybold" in the Melville household
Massachusetts Census, 1855 via Ancestry.com

"With fairest flowers,
Whilst summer lasts, and I live here, Fidele—" --epigraph from Cymbeline 4.2 for "The Piazza," the first story in Melville's The Piazza Tales (1856). 

At Ancestry back in 2014, North Adams genealogist rbass101 posted images 12-13 from the Massachusetts Archives of the state census for 1855 in Berkshire County. Here's image 12 which is accessible by subscription at Ancestry.com. The census of the Melville household (Dwelling #193; Family #201) begins at the bottom of the page:

Massachusetts Census, 1855 / Berkshire County / Pittsfield
via Ancestry.com

Images 12-13 together show who was living there at "Farmer" Melville's full house in Pittsfield: Herman (age 36), wife Elizabeth (age 33), children Malcolm (age 6), Stanwix (age 4), Bessie (age 2) and the baby Fanny (6 months). The 1855 state census of the Arrowhead household lists Herman's mother Maria Gansevoort Melville as "M. S. Melville" (not "Mrs. Melville"?) and gives her age as 63 (in September 1855 she was 64, born April 6, 1791). Herman's younger sisters Augusta Melville, then 33, and Fanny Melville, 29, were there, too.

The last person named in the Melville household:

Fidelia Leybold, 34, of Germany.

Employed in the Melville family around the time of The Piazza Tales, Fidelia Leybold apparently was gone by 1860. As indicated in the 1860 United States Federal Census, the Melvilles by then have a new German housekeeper or "Domestic," Frederika Schmidt of Württemberg. Ms. Schmidt in 1860 is 36 years old with a "Personal Estate" valued at $100. Herman Melville's "Personal Estate" in 1860 is valued at $3000; Real Estate $3500. His stated occupation is "Author."