Yūko Kinoshita is the fraternal twin sister of Hideyoshi Kinoshita, born three minutes earlier. She attends Fumizuki Academy as Class 2-A's Ambassador and ranks among the top ten students academically. Her appearance mirrors Hideyoshi's nearly perfectly, differentiated only by her girls' uniform—a white-and-red shirt with a red tie and skirt—and navy blue hair pins worn on her left side. Yūko displays a prideful, arrogant, and easily agitated personality, clashing with Hideyoshi's calm nature. She habitually mocks him as an "idiot" due to his lower academic performance and extracurricular pursuits. She spends her free time reading yaoi novels.

Her dynamic with Hideyoshi centers on fierce rivalry. She forces him to impersonate her in a school promotional video through threats, exposing her intimidating demeanor and frustration over personal shortcomings like musical ability and rhythm. This inferiority complex erupts when Hideyoshi outperforms her in these areas, causing her to "go berserk." Class F exploits their resemblance by having Hideyoshi pose as Yūko to provoke other classes, escalating inter-class tensions. After the Summoner Test War between Class F and Class A ends in a draw—during which Yūko falls into a pit but her Summoned Being later claims victory—she holds a deep grudge against Hideyoshi and Class F for the incident.

Her Summoned Being wears heavy armor and carries a jousting lance. During a Summoning Field system error, it transforms into a Bakeneko, a ghost cat reflecting its deceptive capabilities; this occult form is exclusive to the anime adaptation.

Her name "Yūko" (優子) blends "excellence/superiority" (優) and "child" (子), while "Kinoshita" (木下) means "under the wood/tree." She shares a birthday with Hideyoshi and Norwegian DJ Alan Walker.