Yukika is a friend of Kane Himuro and Kaede Makidera, known together as Class 2-A's famed track girl trio. She is the gentle manager of the Track Team, warmly loved by the majority of her class, and the mascot girl of the trio. Originally she was supposed to join the Cooking Club, but she became the manager of the Cross-country Club at Makidera's insistence.
Yukika is a shy, patient, and mature girl. Although she lives in poverty, her heart shines brighter than the most lustrous silk. Consequently, her smile heals those around her. While she is not an honor student, she respects and looks up to her classmate Rin Tohsaka in awe. She has several younger brothers of various ages. She's very affectionate towards her younger brothers and she spends a lot of her free time looking after them. She is extremely bad at sports.
Fate/Grand Order x Himuro's Universe: Seven Most Powerful Great Figures Chapter
Yukika is one of the few people able to see Assassin at the Ryuudou Temple gate, and strikes up a conversation with him in Fate/hollow ataraxia. This is because, although unaware of it herself, she has psychic abilities, allowing her to see spirits and sometimes she can't distinguish between real people and spirits because of how often she sees them.
 Fate/side material - Encyclopedia: Saegusa Yukika [Person's name], p.061 [T]
Yukika Saegusa [Person's name]
A female student of Second year Class A.
The mascot of the girl trio. She was quite personable, patient, and mature.
She was not an honour student, and she looked up to her classmate Tohsaka Rin in awe.
She had several younger brothers of various ages.
Although she lived in poverty, her heart shone brighter than the most lustrous silk.
Consequently, her smile healed those around her, and her rabbit-like awwww-inducing qualities really stirred the protective instinct in people.
Originally she supposed to join the Cooking Club, but she became the manager of the Cross-country Club at Makidera's insistence.
By the way, just like you thought, she was extremely bad at sports.
- Fate/Grand Order x Himuro's Universe: Seven Most Powerful Great Figures Chapter
- TYPE-MOON Gakuen Chibichuki - Chapter 7.5