Spoiler warning: Plot and/or ending details follow.
A xenopsychologist by trade, Petty Officer 2nd Class (Dr.) Reiko Onami, Ph.D. was a Human Starfleet enlisted crewman who served aboard the USS Enterprise in the 2270s as a member of the medical staff. She was recruited by Captain Willard Decker to serve as a counselor for the ship's multi-species crew. (TOS novel: Ex Machina, TOS movie, novelization & comic adaptation: The Motion Picture)
Dr. Onami was raised on Nelgha, a planet outside the Federation, by the only two humans in the sector. This upbringing allowed her to understand and even befriend most aliens she'd ever met, even the scoundrels and brutes; however, it also left her struggling to relate to other humans.
When Rear Admiral James T. Kirk and Dr. Leonard McCoy returned to the Enterprise in 2273, Dr. Onami was less-than-pleased, having grown quite fond of Captain Decker and was repulsed by Dr. McCoy's seemingly insensitive attitudes toward the multi-species crew, as well as indignation on behalf of her friend, now-Deputy Chief Medical Officer Christine Chapel, who was a qualified xeno-medicine specialist. However, after a few months of serving together, Dr. Onami and McCoy grew to be friends, sharing similar points of view regarding Starfleet, the Federation, and even the human race. (TOS novel: Ex Machina)
By 2278, Dr. Onami had been promoted to Chief Petty Officer. Upon the conclusion of the Enterprise's five-year mission, she took a leave of absence from Starfleet to study the New Human movement. (TOS novel: The Higher Frontier)