Even though there are many rumors about him being a delinquent, she's not scared of him when he talks to her. They get along well from the very beginning and is often seen together with him. Both of Chiba and Yukko are supporting Sakura's love for Kuzumi and helping her.
Yukko admitted to him that she doesn't understand the feeling of love, like Erika, and she never fell in love with someone.
Once, Yukko told Chiba that she likes him. He didn't get it and thought it was an act of pity because nobody have never told him they like him. Since then, they are always together and Yukko blushes often over Chiba's acts and words.
Yukko and Erika know each other since middle school. They passed the entrance exam for their high school together. Then, Yukko was happy and told Erika she will be able to fool around with her for three more years.
Since Yukko noticed Erika's love for Kuzumi, she can't stop teasing her about that and ever took pictures of her sleeping with Kuzumi at night during school trip.