Shiro: For Shiro, Kiyotaka was a very good classmate of his and is much stronger than him. This is also why he had offered Kiyotaka to escape the White Room with him. However, even though Shiro respects Kiyotaka a lot, he doesn't think of him as a friend. He won't do anything that only has a benefit for Kiyotaka and not him. If he invites Kiyotaka somewhere, he would usually only invite him because Yuki asked(or forced) him to. He also has a little desire to surpass him even though he already knows that it's almost completely impossible for him to do so.
Yuki: Yuki is in love with Kiyotaka. She has been like this ever since she was 5 years old. Her main motivation for wanting to stay in the White Room was so that she could continue being with Kiyotaka. This is also why she wanted to see Kiyotaka after the White Room had shut down even though he didn't even try to help her back when the instructors wanted to take her away. After reuniting with Kiyotaka once again, Yuki was very happy to be with him again and vowed to herself that she would make Kiyotaka fall in love with her, no matter what.