This street appears to be named for the British writer William Makepeace Thackeray (1811–1863), best known for his 1848 novel Vanity Fair. As local historian Rob Ketcherside explains in his blog post on “Renaming streets of Seattle’s Fremont to the U District”:
42nd St, 50th St, 1st Ave, and 5th Ave are the boundaries of the 1889 Harrison Heights Addition to the City of Seattle (annexed in 1891). J.A. Gould and Anna L. Gould, the owners, had the civil engineers draw a simple box around their property and didn’t bother to make their streets match those adjacent. But when the streets were renamed and woven together in the 1890s, 1st and 2nd Avenues bent west at 42nd while Latona, 4th and 5th bent east. Space for an extra road was left in between: Thackeray Place.
Their Dickens Street became 1st Avenue NE; Scott Street, 2nd Avenue NE; Cooper Street, Latona Avenue NE; Milton Street, 4th Avenue NE; and Kingsley Street, 5th Avenue NE. Thackery Street, as it was originally named, had no corresponding street in the already existing grid. (Why it was left alone instead of being renamed something like 2nd Place NE is unclear.) As Rob points out, these names are almost certainly those of the writers Charles Dickens, Walter Scott, James Fenimore Cooper, John Milton, Charles Kingsley… and William Makepeace Thackeray.
(J.A. (Joel Abdon) Gould, incidentally, appears to have been the founder of the first bank in San Juan County, the aptly named San Juan County Bank in Friday Harbor. His wife’s full name was Anna Lucretia Cary Gould; their son, Eugene Cary Gould Sr., succeeded his father as bank president and was also the first mayor of Friday Harbor.)
Thackeray Place NE begins at NE 42nd Street and goes ½ a mile north to NE 50th Street.