Kate was born in New York where her parents Rosemary Boothe and Seamus O'Sullivan ran a pub that dated its history back to ancient New York. The pub was know for many years as Molly's. The O'Sullivan's home was upstairs from the bar on Third and Twenty-third sts. The O'Sullivan's were members of the ancient parish of St. Patrick's where the family devoutly worshiped. Early in childhood, Kate was pushed by her parents to pursue a life of culture an education. It was hoped that she would pursue a religious life as well, devoting her heart and soul to a life of chastity and service.

Bucking the constraints of this idealistic life, 2267 she entered the Academy as a student in the school of Nursing. She graduated in 2271 in the top 10% of her class and was immediately assigned to the Utopia Planetia Fleet Yards on Mars where she worked as a floor nurse in the infirmary. This was her first trip off world other then the required training cruise while in her fourth year of school. In 2278 she was posted to the USS Bozeman as head nurse in charge of a staff of five other nurses. Three weeks out of starbase the Bozeman was lost in the Thyphan Expanse. She had been thrown into a causality loop only to emerge ninety years later, in 2368. No one in the crew had been aware of what had occurred. Kate choose to remain in Starfleet and re-enrolled in the Academy to update her training. In 2370 she was assigned to the fleet base on Ganymead as a staff nurse. In 2373 she was posted to the T'nar outpost near the Cardassian boarder, during which time the outpost was subjected to an attack by the Maquis. She served aboard the USS Enterprise-E from 2374-2377. She has since served as head Nurse on the USS Endeavour.

Kate can be described as a strong willed individual with a lot of inner reserve. She tends to be very professional in her attitude. Off duty, she is warm and compassionate with her friends and crewmates. She listens well and tires to always lend an ear to those who need it. She responds well in an emergency, keeping calm and level headed, she is not prone to panic. She tends to bite back her anger but occasionally looses it in a good Irish fit.

As for personal relationships, Kate wed just prior to shipping out on the Bozeman. Her new husband (Emil Henri DuBec Gessard) was a commercial shuttle pilot doing the run between Earth and the Jovain system. He had been married previously and had suffered through a bitter divorce. He ended up with custody of his two young daughters (Adele 5, Yvette, 3). Kate cared intensely about the girls and thought of them as her own. She and Emil were very devoted to each other despite the pressures of their individual careers, and the frequency of when they saw each other. They spent short but happy hours together when they could. Upon the Bozeman's return, she was going to request assignment to either the base on Ganymead or to Fleet Headquarters in San Francisco to keep the family together.

In 2380 she married Peter Allen MacLaughlin, Captain of the USS Lovell. Peter was killed on the Khynah homeworld, Den'a in 2381 assisting other prisoners to escape.

She swims and does ballet to stay in shape. She plays the guitar, recorder and flute. She also took up painting in watercolors and drawing as part of the therapy she was put through to adjust her to her new life in the Twenty-fourth century. She can often be found hanging out with her friends in a lounge or in the commissary, occasionally playing cards. She does not enjoy the holodeck (due in part to a training accident), nor does she play any team sports.