(written from a Production point of view)
Kellam de Forest (born 11 November 1926; age 90) is a researcher who worked on Star Trek: The Original Series. His one-man operation, The Kellam de Forest Research Company was hired by Gene Roddenberry to verify facts and correct errors in stories, starting with the first pilot, "The Cage". He never received any screen credit for his work.
Among the facts de Forest corrected was the Native American tribes whose mixture the inhabitants were in "The Paradise Syndrome". He furthermore corrected the costumes and props of the Indians accordingly, but they were not changed in the filmed episode. (Star Trek and History: Race-ing Toward a White Future) He also came up with the idea of silicon-based life for "The Devil in the Dark", as the original chemical substance in Gene L. Coon's script deemed to be inaccurate. (Inside Star Trek - The Real Story) Possibly, de Forest made the suggestion to change the Latin names of ancient gods to their original Greek ones, in the script of "Who Mourns for Adonais?". 
Apart his work on Star Trek, de Forest worked as researcher on other television projects, including The Untouchables, which was also produced by Desilu. He was also the researcher on the pilot of William Shatner's series Barbary Coast, which also featured Michael Ansara. He also wrote the story for an episode of The Adventures of Jim Bowie (which featured Whit Bissell) and two episodes of Yancy Derringer (one of them featured Roy Jenson, the other Bert Remsen and Willard Sage).
In 1998, de Forest was interviewed in the documentary Inside Star Trek - The Real Story.
Today, de Forest lives in his birthplace of Santa Barbara, California, and is mainly occupied with landscape preservation, maintaining the legacy of his father (landscape architect Lockwood de Forest).