Killer Kidd was a male Human of 20th century Earth. He was a professional athlete who participated in boxing contests.
In 1930, he fought against Gus Lloyd as part of the preliminary fights of two boxing evenings, in Madison Square Garden in New York City and Bay Land Garden in San Francisco. (TOS: "The City on the Edge of Forever"; DS9: "Past Tense, Part II")
This character was only mentioned in writing.