She seems to have a close relationship with Yōsuke as when the school first started she cheered on his idea of the class introducing themselves.
After several days of school, they seem to have formed a bond as they are often seen together. In truth, they're just pretending to be in a relationship in order for Kei to not get bullied in school so it is unknown how they really feel about each other.
In the light novel of Volume 4, despite being promised to protect Karuizawa from being bullied by Manabe and her group, Yōsuke doesn't want to take a side and want to discuss it peacefully without using violence against someone. He did offer his help to her, but since he's not her real boyfriend, he didn't need to go that far for her. He also tells her that he won't help her like this and she won't be able to depend on him if there's nothing he can do for her.
They don't have much interaction but don't appear on good terms.
In episode 7, she was rather callous to her and told Yōsuke they should go.
During the survival test, after Yōsuke is chosen by Kei to watch over the boys in the class, Suzune steps in. She doesn't oppose Kei's request of watching the boys but requested another boy join him on guard duty since he needed to be watched as well. After Kei mentions there was no other boy who seemed trustworthy, Suzune selected Kiyotaka.
During the survival test, when Yōsuke is chosen to watch over the boys in the class, Suzune requested Kiyotaka join him on guard duty since he needed to be watched as well. While he is picked, Kei notes on him being invisible to the rest of the class, showing that she barely acknowledges his presence in their time as classmates.
Due to his real thoughts, it is unknown what he really thinks of her.
Much like the anime, Kei initially did not think much of Kiyotaka. She often looked down on him for being an "unremarkable guy" and appeared disgusted by his presence when they were both placed in the "Rabbit" group during the summer special test, although she shared the same feelings about her other groupmates, Hideo and Teruhiko, so it wasn't personal.
After being saved from Manabe and her group during the cruise ship test, she starts to hold a dislike towards Kiyotaka, unable to look at him without an angry expression and was mad when Hirata arranged a meeting with her but brought Kiyotaka with him. After being bullied by Manabe and her group in an isolated ward, she was blackmailed by Kiyotaka and threatened her that he'll reveal her past to the school, but assured her that he'll protect her from being bullied in exchange for aiding him on his scheme. 
She was amazed at his true physical abilities at the sports festival after he raced Manabu Horikita and nearly won. She seems to have formed a friendship with him which evolves into a close trusting bond with him, where Kiyotaka is shown to truly trust her more so than others, even more than Suzune and in volume 6 of the light novel, it seems that she develops a crush on Kiyotaka. Her crush on him was also hinted further in the light novel when she became jealous of Maya's flirtation with Kiyotaka. Kei is one of the people who refer Kiyotaka on the first-name basis. She was also the first person to celebrate his birthday, causing him to hesitate on deleting her text messages, which he usually does. He also learned of her birthday and decided to do the same for her.
In Volume 7, even though she learned from Kakeru Ryūen that Kiyotaka betrayed her by using her from the very beginning, she still refused to betray Kiyotaka, calling him her hero, as she refused to tell Kakeru Ryūen that Kiyotaka is the true mastermind of Class D. However, she was surprised to see Kiyotaka beating up Ryūen's gang, and angrily attack Ryūen for threatening her. She was shaken by his abilities but hugged him as she was relieved that he came to her aid as he mentioned his promise to protect her.
In her monologue in volume 7.5, she admits that she has fallen in love with him.
In volume 11.5, Kei sees Kiyotaka and Hiyori Shiina going on a date and becomes jealous. She is also jealous that Kiyotaka has the same hobby as Hiyori.
In the epilogue of volume 11.5, Kiyotaka declares his love for her and she accepts.
Kei and Kikyō appear to be on good terms. In episode 2, Kei went to Kikyō for points as she was out of it after blowing through her own funds and seemed desperate for points as she used their friendship as an excuse to do so but it appears Kikyō didn't go through with it.
Satsuki is one of Kei's friends in class 1-D. The two can often be seen interacting with each other in the anime and getting along very well.
During the survival test, Satsuki was among the many girls that took up for Kei after her underwear was stolen.
Maya Satō is one of Kei's friends in class 1-D, as they get along well. The two can often be seen interacting with each other in the anime and getting along very well.
During the survival test, Maya was among the many girls that took up for Kei after her underwear was stolen.
In Volume 6 of the light novel, Kei becomes jealous of Maya's flirtation to Kiyotaka and their close relationship. This causes her to consider her feelings for Kiyotaka.
In Volume 11.5 Maya feels like Kiyotaka likes Kei and thinks that Kei might have abandoned Hirata to be with him.
Chiaki Matsushita is one of Kei's friends in class 1-D.
She notices that Kei often looks at Kiyotaka. Previously she ignored this because Kei had a boyfriend but after she broke up with Hirata, she now becomes suspicious of it.
- ↑ 1.0 1.1 You-Zitsu Light Novel — Volume 4
- ↑ You-Zitsu Light Novel — Volume 5
- ↑ You-Zitsu Light Novel — Volume 6
- ↑ You-Zitsu Light Novel — Volume 7
- ↑ You-Zitsu Light Novel — Volume 7.5