On his homeworld of Sadr II, D'Hennish was a prince and, as such, had a docked tail. He was two meters tall, slender and wiry, and had short platinum-colored fur with a longer ash-blond mane. His eyes were a brown-amber color with pupils that could change shape from slits to circles like a cat's. He also had a throaty voice.
In the year 2275, D'Hennish rewrote a cartographic program as part of the Enterprise's tests of K't'lk's inversion drive. During the drive's testing, he posited to Captain Kirk that entropy was no longer occurring in the Lesser Magellanic galaxy.
Later in the testing, when a non-entropic universe was discovered to have breached into the Lesser Magellanic galaxy, D'Hennish, Spock and Uhura communicated with the creature at the centre of that universe and gave it self-awareness. This allowed the creature to be reasoned with, and ultimately mend the universal barrier. (TOS novel: The Wounded Sky)