Yukiko Kudo is married to Yusaku Kudo, and is the mother of Shinichi Kudo. She had become a famous actress and idol at a very young age, but abandoned her promising career in order to marry her husband. This caused an uproar within her family at the time, since she had been only 20 years old. After having Jimmy, she and her husband traveled to America to pursue their own careers after he had grown old enough to care for himself.
Yukiko is also an old friend of Ran Mouri mother Eri Kisaki from high school, and both had competed in a pageant which had taken place in their school. Despite still being friends, they keep up a friendly competition in those regards, since the contest had ended in a tie. (Kogoro had failed to turn in his vote.)
In episode 43, Yukiko makes her first appearance in the series. In the episode, the couple had returned from America to discover that their son had gone missing. Dr. Hiroshi Agasa feels inclined to tell them about what had happened to Shinichi, and the danger that he is in. They are extremely worried about him and want him to go live with them in America. They do this by staging a mock "kidnapping" of one Conan Edogawa and create a case for him to solve, while scaring him half to death at the same time.
Yukiko disguises herself as Conan's supposed mother, "Fanny Edogawa" and shows up at the Moore Detective Agency to pick him up. Conan immediately knows something is wrong since Conan was an alias that he made up, and fears her to be part of the Black Organization. In the end, she and her husband reveal themselves to him, and plead with him to stay with them in America. Conan rejects the idea, because he feels that he needs to bring down the Black Organization and keep the Professor, Ran, and her family safe at the same time. Thus, Yukiko takes him back to Ran Mouri place while disguised as Fanny. She and her husband then return to America, but check in on him from time to time.
Later, in episode 96, Yukiko returns to Japan to find that Ran is highly suspicious of Conan being Shinichi. Yukiko pretends that she didn't know that Conan is Shinichi to lure Ran away, and convinces her that Conan is a very distant relative of both herself and the professor. Later in the episode, Yukiko takes Conan to her friend's house, where yet another mystery has to be solved.
Yukiko had also studied for an acting role with her friend, Sharon Vineyard (who unbeknowest to her, was Vermouth) under the world-famous stage magician, Toichi Kuroba. One of the things he taught them was disguise, and they both become masters of the art. They did not know that Toichi Kuroba was secretly the original Kaitou Kid, who was later murdered by the Black Organization, and whose identity was later discovered and taken up by his own son. Yukiko had also met the successor of Kid, yet she also does not know. However, what she did know was that she had met Toichi's son before, Kuroba Kaito when he was 10.
Yukiko's personality is more that of a wild, over-confident teenager than of a mature woman; she is impulsive, a reckless driver (caught back in New York because of it), and likes to stand in the limelight (literally) whenever she has the chance. Because of her marriage to a famous mystery author and her knack for becoming involved in criminal cases from time to time, she has been publicly dubbed the "Night Baroness", after one of her husband's more notorious characters, the Night Baron.
A running gag in the series is that whenever Yukiko meets her son and they have to pretend not to know each other, in the original Japanese versions Shinichi (or rather, Conan) calls her "oba-san". This term, which means "old lady" or "grandmother", is often used by small children for women in their thirties and forties, sometimes even younger ones - a rather discomforting thought for those Japanese women who consider themselves in the prime of their lives. Yukiko also reacts rather unfavorably to it - she is "only" 37 years old - so Conan has to correct himself by calling her "onēe-san" ("older sister").