She is the oldest Gate Keeper in Japan, born in 1169. However, she has ceased aging and appears as a young girl in a white kimono in 1969. Ruriko and Shun first encountered her during their mission to recover a crashed plane and its cargo at Mt. Daisetsu in GK. As her name Yukino suggests, she commands the Gate of Ice and Snow, capable of summoning blizzards and hail. Yukino can also teleport and send visions that allow people to witness scenes without being physically present. Reiji Kageyama dubs her the most powerful Gate Keeper in the series. One of her unique traits is her habit of conversing in Tanka poetry. Though she assists A.E.G.I.S. members, she is not an official member. She owns a pet ermine named Hisame and is closest to Shun's sister, Saemi Ukiya.
In GK21, Yukino reappeared, taking up residence in Tategami High and becoming the source of rumors about nighttime ghosts. Tragically, while her body hasn't aged, her powers and strength are waning. She long ago stopped appearing before her old friend, Saemi. Recognizing a similar sadness in Ayane, the daughter of an old friend, she helps and befriends Ayane and Miu.
The reason for her agelessness is explained in episode 5 of GK21. Gate Keepers often have sad backgrounds because their gates absorb their happiness. Reiji Kageyama (Nazo) mentions that frequent use of gates can destroy a gatekeeper. In Yukino’s case, she constantly summoned her gate in emergencies, causing time to freeze due to overuse. She shared a close relationship with Reiji, discussing a promise they had made in one of the OVA scenes.
After sustaining a fatal wound in her confrontation with Yuurei Shoujo in GK21, her body dissipated into snow. However, she reappeared in the final scene, picking up Hisame and looking just as she did before.
