Kirayoshi O'Brien was born in 2373, the second child of Miles and Keiko O'Brien. Kira Nerys acted as a surrogate mother for Kirayoshi after Keiko was injured during a mission to the Gamma Quadrant in late 2372, carrying the baby to term. Kirayoshi's name was often shortened to "Yoshi."
Conception and birth
Kirayoshi was conceived in 2372. Miles found out Keiko was pregnant again after Keiko and Molly arrived back from a trip to Bajor. It was Molly who told her father that she was going to have a little brother. Miles initially believed she was talking about her doll, but, after Molly pointed to Keiko's stomach, he realized what she meant. (DS9: "Accession")
The news quickly spread around Deep Space 9. Julian Bashir gave his congratulations and the expecting parents and celebrated with a drink in Quark's. Worf also learned of the news and, recalling his reluctant role during the birth of Molly, told O'Brien that he would be away from DS9 at the time Kirayoshi was born ("Far away. Visiting my parents. On Earth."). (DS9: "Accession")
Several months later, Keiko and Kira Nerys were involved in an accident aboard a runabout in which Keiko was critically injured. In order to save both mother and unborn child, Julian Bashir transferred the baby to an undamaged womb – namely Kira's. Although the procedure required various hormonal injections in order to allow Kira's Bajoran body chemistry to cope with a human infant, it proved generally successful, but due to the dense blood vessels that form between the mother and baby during a Bajoran pregnancy, Kira would have to carry the baby to term as a transfer back to Keiko would prove too dangerous for Kira and the baby. Thereafter, Kira moved in with the O'Briens so that they could continue to feel close to their unborn child. (DS9: "Body Parts")
The Cardassian Silaran Prin intended to remove Kirayoshi from Kira's womb several weeks before he was due to be born, wishing to differentiate himself from Kira's old resistance movement by refusing to harm the 'innocent'. Fortunately, the makara herbs she had been taking counteracted the effects of Prin's merfadon, which enabled Kira to stop him. (DS9: "The Darkness and the Light")
Kirayoshi was delivered in a traditional Bajoran birthing ceremony in early 2373. Miles, Keiko, Shakaar Edon, and Y'Pora, a Bajoran midwife, were at her side; Kira later expressed some sorrow that she was not able to raise the baby herself, having genuinely bonded with the child during her time as a surrogate. (DS9: "The Begotten")
Several weeks after the birth, Kira claimed that Kirayoshi recognized her, although a skeptical Jadzia Dax believed he was too young to "even recognize his own fingers." Even after Kira told her that Kirayoshi smiled every time she entered the room, Jadzia retorted that it was "probably just gas." (DS9: "In Purgatory's Shadow")
Later that year Keiko had to go to Bajor, leaving Miles to look after Kirayoshi and Molly. He found that no one could look after Yoshi without the baby bursting into tears. Jake Sisko was one such unsuccessful babysitter. Miles later found that Yoshi slept better in the pit in DS9's operations center. He believed it was the "pattern of lights... the hum of the optronic emitters, or the vibration of the ODN relays". Miles later found that Worf had a calming effect on Kirayoshi; Worf would sing Klingon nursery rhymes to Kirayoshi while babysitting him. (DS9: "Business as Usual")
Tekeny Ghemor thought Yoshi was "a beautiful baby" when he visited the station just before his death in 2373. Owing to his complicated fatherlike relationship with Kira, he said he felt that, in a strange way, Yoshi was the closest thing he had to a grandson. (DS9: "Ties of Blood and Water")
When Molly O'Brien was lost in time in 2374, Worf took care of Kirayoshi, in an effort to prove to Jadzia that he was well suited as a father, as Miles and Keiko tried to reintegrate Molly into the family and station life. Although Worf felt briefly disheartened after he accidentally bumped Kirayoshi's head during a Klingon game, his mood improved when he learned that Kirayoshi was enthusiastically repeating the phrase "gung-gung-gung", part of a Klingon exercise called Hehh-duHpp that was traditionally taught to Klingon children that Worf had played with Kirayoshi during his time babysitting (DS9: "Time's Orphan").