Class 1-C Edit
For unknown reasons, Mio is always by his side as one of his followers, however, it is clear she doesn't enjoy the pain he inflicts on others as shown when she flinched after he smashed a glass bottle on the head of Daichi or when he had Albert beat Daichi and the others for their failure in getting Ken expelled. On the cruise, she angrily confronted him and made it clear that she couldn't stand nor approve of their cruelty and wanted them to stop their actions. She also doesn't tolerate him ignoring her words or treating her like a plaything when he made advances on her. After he tried to leave, she called out to him but Albert intervened and Mio was angry and disappointed that he manipulated him into being his pawn before Albert threw her down and Kakeru grinned at her pain.
It appears this was merely an act to get D-Class to believe Mio disliked their ways, as in the survival test Kakeru had Mio hurt to ensure she sneaks into their camp and discovers their leader. Mio even apologized for letting Kakeru down but he shrugs it off as there was someone in D-Class who deceived them.
While nothing is known about their relationship, it appears that she knows him very well as she was shocked that he would grab her and throw her on the ground after she tried to confront Kakeru. Mio voiced her disappointment in him for becoming a follower of Kakeru as he looked at her with an unfazed expression and walked off.
Class 1-D Edit
The two first met on the school cruise where he witnesses her being knocked down by Albert after being shunned by Kakeru. Kiyotaka proceeds to help her up but she angrily slaps away his hand telling him to stay out of her business before running off.
They met again during the survival test where he found her hurt for unknown reasons, even finding himself stopping Haruki from approaching her, due to having suspicions about her, but soon says it was nothing, as they offer her help by letting her come to their base camp. Though she refuses help from an enemy class, she accepts their offer and returns to their camp but makes it clear she isn't trying to be friendly with them.
After the panty theft incident, Mio later talks with Kiyotaka asking if he believes she was responsible for it but he says that he knows she didn't do it, to her surprise. He states that while the other students in his class don't trust her, he does, which causes her to blush and nervously thank him for having belief in her. She was also seen curious, as she glanced back from behind, when Kikyō and Haruki commented on how incredibly close Suzune and Kiyotaka have gotten lately.
After the survival test, it is revealed that Kiyotaka knew she was a spy and he outsmarted her by using various tactics which helped his class pass the test.
In light novels, both her and Kiyotaka were seen together wearing casual looking clothes and heading for a fortune teller.
When he revealed he was the mastermind of D-Class, she disbelieved him as she considered him to be a simply nice guy but was shocked to learn that it was true after he revealed that he was the one who sabotaged her plans in the survival test. She was further shocked when seeing him take down Albert and Daichi with ease by beating them down becoming speechless at his abilities. She similarly charged at him as he ignored her questions about him being D-Class' manipulator and she soon admitted she just couldn't believe it was him before she was instantly beaten and knocked out cold by Kiyotaka with no problem, making him victorious over her.
In their next meetings, things are really awkward since Mio is now aware of his abilities and intellect while unable to understand his logic.
Not much is known about their relationship, but it seems that they have a rivalry going on. In the survival test, Mio was seen, after she was revealed to have stolen the key card, fighting Suzune and easily overpowering her due to the latter's illness, which caused Mio to forcibly defeat her. 
- ↑ You-Zitsu Light Novel and Anime — Volume 2, and Episode 4
- ↑ 2.0 2.1 2.2 You-Zitsu Anime — Episode 8
- ↑ You-Zitsu Light Novel and Anime — Volume 3, and Episode 12
- ↑ You-Zitsu Anime — Episode 9
- ↑ 5.0 5.1 5.2 You-Zitsu Light Novel and Anime — Volume 3, and Episode 11
- ↑ You-Zitsu Light Novel — Volume 4.5
- ↑ You-Zitsu Light Novel — Volume 7