Starfleet botanist who has served aboard the U.S.S. Enterprise-D, Deep Space 9 and the planet Bajor. Born Keiko Ishikawa, she is the wife of Starfleet officer Miles O'Brien. They got married in 2367 aboard the Enterprise-D in a ceremony that combined Japanese and Irish traditions. That wedding took place in the ship's Ten-Forward lounge, which is where, a year later, Keiko gave birth to their first child, Molly, during a ship-wide systems failure.

When her husband was assigned to Deep Space 9 as chief of operations in 2369, Keiko accepted a transfer to the same outpost. Keiko found adapting to life on the station difficult. She was dissatisfied by the lack of professional opportunities on the station for a botanist, and was concerned about the lack of educational facilities for her daughter. She therefore requested permission to establish a schoolroom on the station, which Commander Sisko granted. Keiko operated the school until early 2371, when enrollment dropped dramatically because of fears of a possible Dominion attack. Shortly thereafter she accepted an assignment as chief botanist on an agrobiology expedition to Bajor. The assignment kept Keiko and her daughter, Molly, away from the station for the better part of a year. In 2372, Keiko became pregnant with her second child, and after completing her assignment on Bajor, she and Molly returned to live on DS9. But Keiko's second pregnancy nearly ended disastrously when she was seriously injured in an accident aboard a runabout during a botanical survey mission. Dr. Bashir saved her life, but it was necessary to remove the unborn child from her womb and implant it into the body of Kira Nerys, who carried it to term. Keiko and Miles invited Kira to live with them in their quarters so they could care for her during the pregnancy, and in 2373, Kirayoshi O'Brien — a boy — was born in a traditional Bajoran birthing ceremony.

In 2375, upon the end of the Dominion War, Keiko, Molly and Yoshi moved to Earth when Miles took a job as instructor at Starfleet Academy.

Keiko grew up with strong Japanese traditions and has always been close to her extended family. As a child, Keiko helped her grandmother with her Japanese brush painting; it was her special task to fill the old chipped cup that her grandmother — whom she called Obachan (a Japanese term of endearment for an older woman) — used to clean the brush. In 2369 Keiko and Miles took a break from duties aboard DS9 to visit Earth for her mother's 100th birthday.