Crown Princess Kareen was the mother of now (as of Diplomatic Immunity) Emperor Gregor, and the wife of Crown Prince Serg. Prior to her husband's death she encouraged a not-entirely platonic relationship with Count Vidal Vordarian, hoping that upon the death of her father-in-law, Emperor Ezar Vorbarra, Count Vordarian would be able to protect her from her husband Serg's cruelty and depravities.
Upon the death of Prince Serg, Count Vordarian first sought to insinuate himself into power through his open courtship of the now Dowager Crown Princess Kareen. Kareen for her part, sought to let the relationship cool, as she no longer needed protection from her husband.
When Emperor Ezar died, Count Vordarian first sought to again exploit his relationship with Kareen to have himself appointed as Regent of Barrayar. When Aral Vorkosigan, Ezar's choice, was appointed by the Council of Counts, Vordarian and his conservative Vor allies fomented a coup d'état, since called Vordarian's Pretendership, in which he seized the Imperial Residence and sought to marry Kareen and have himself declared Regent for her son, the child-emperor Gregor. Loyalists were able to rescue Gregor from the residence, although Vordarian led Kareen to believe he had been killed in the attempt. At that point, Vordarian declared himself Emperor, and compelled Kareen to share his bed, with the intentions of marrying her to cement his claim to the throne.
Kareen was killed by a guard loyal to Vordarian when Cordelia Naismith was able to gain entry into the Imperial Residence and convince Kareen that her son had survived, at which point the emotionally wrought Kareen leveled a weapon at Vordarian.
Princess Kareen was described as pale, even wan, woman with a slender frame, dark eyes and beautiful dark hair that was always highly coiffed and elegant. She was in her early thirties at the time of her death.
Ludmilla "Drou" Droushnakovi was her personal bodyguard before Kareen transferred her services to Cordelia Vorkosigan. "Drou"'s daughter Kareen is named after the Princess.