Following an altercation with Shizuo, Ruri still in her Hollywood costume, lands in front of Kasuka's car on a desolate alleyway. Unconscious and injured, Kasuka decides to ferry her to his apartment rather than a hospital. Once there, he contacts Shinra to treat Ruri in order to avoid exposing her identity to her pursuers. Ruri is initially hostile and distrustful of Kasuka, but quickly warms up to him once she sees his concern is genuine.
When the two of them decided to become a couple it was initially meant to be a temporary ruse in order to rile up the reporters that had surrounded the entrance to Kasuka's apartment as a way to keep Nebula's employees from getting close to Ruri. However, after all was said and done, Ruri intended to tell Kasuka the story of her family and then commit suicide. Kasuka apparently picked up on this sentiment and refused to hear Ruri's story, saying that he didn't want Ruri to die. The two of them then decided to continue their relationship and began to grow closer to one another with Ruri even deciding to work at Kasuka's studio.
When Kasuka first offers aid to Ruri, he does so out of a sense of responsibility due to the fact that it was his brother that injured her as well as a sense of chivalry that Kasuka adheres to. As their relationship continues, Kasuka shows a protective streak towards Ruri especially after her home was burned down by Kisuke Adabashi.
Ruri was perplexed by Kasuka's nature and was intrigued as to how he stayed so emotionless and detached on the outside yet at the same time seemed to emanate so much emotion when he was acting on camera. Indeed, it appears to be the quality she admires most about him and is what drew her to him as they spent more time together.
The only real source of conflict regarding their relationship would probably be Ruri's apparent fear of Shizuo. Since Shizuo was able to defeat her so easily and knock her halfway across the city, she has developed a sort of phobia towards him. Kasuka seems to be rather understanding of this considering that Shizuo has this kind of effect on almost everyone.