Initially, Erika was no more than a pet dog to Kyouya. However, later on in the series, he realizes his feelings for her and they officially became a couple. He still often teases her and calls her "annoying" at times but he remains extremely loyal to Erika. Kyouya also developed a jealous and protective attitude towards Erika whenever she's associating with other guys. This is shown when he is upset over the fact that Erika gave Yuu Kusakabe some cupcakes for Valentines which irritated him. Despite his tough front, he loves Erika very much, as in episode 8, he tries to say "I love you" to her during Erika's birthday date. In the end, he does say it by whispering into her ears. In Chapter 43, Kyouya beats Terasaki (Terapon) for hitting Erika and admits that Erika belongs to him. He also tells Terasaki (Terapon) that if he wants Erika, he must ask him for permission. He then says to Erika to not concern with any other guys anymore and should only look at him. You can also tell that Kyouya is willing to do anything for Erika as in Chapter 47, he plans Erika's birthday for her birthday present. Erika is very important to him, and he is very loyal and faithful towards her. When Erika's cousin, Rena, tells him she's in love with him, he lets her down gently saying that it isn't because he's can't be interested in her but because he doesn't want to be separated from Erika. Later in the manga, they consummate their relationship, although in the beginning when they tried to be intimate, they were either interrupted by his friends or Erika was on her period. In the end the two got married and had a daughter named Yuina.
Ayumi often helps him with matters concerning Erika and they are seen interacting with each other on multiple occasions. It is shown that Ayumi and Erika are really close. However in the OVA and Kyouya's birthday arc they accidentally kiss and this frustrates and makes Erika really worried throughout that arc however Ayumi tells Erika that it was not her first kiss and this shocks Erika,
They first met on the first day of their second year semester where Kamiya was blocking the entrance to the classroom, and the only way to go in was for the girls to tell him their names. At first, Kamiya thought Kyouya was just like him so he forced him to date other girls, but in the end, Kyouya tells him he was the same before he met Erika. Along with Erika and Ayumi, they're often seen hanging out together both during and outside of school hours. Kyouya hasn't mentioned if he thinks they're friends but Kamiya considers himself Kyouya's best friend.
He has a strained relationship with his mother due to the fact that she's been living apart from him a young age. He doesn't visit her in Kobe often and does not interact much with her even if he does. However, his relationship with his mother has improved ever since his last trip to Kobe with Erika. He has also promised to visit her more often.
Takeru and Kyouya have been friends since middle school. Takeru always worries about Kyouya and helps his relationship with Erika as in Chapter 29 he gave Erika and Kyouya some alone time together.
Yukari Kasai (Female Class Rep)
Later chapters in the manga a new girl is selected to become the female class rep alongside Kyouya and the two of them start to meet a lot more in school at school council meetings and after school meetings and he tries to play the princely act with her but she catches him out and tells him to "cut the crap" and to act like he does when he is with Erika and this surprises Kyouya as he finds it refreshing that another girl besides Erika manages to see his true side however this makes Erika really jealous as she starts to see that Kasai has feelings for Kyouya and tries to snatch him away and gets all feely touchy however in the end Kasai is rejected as Kyouya tells her that "although she has the same personality as him and that Erika might be dense and dumb his life would be pretty boring without her" and Kasai is inspired by this making her want to fall in love properly however she decides to back off realising that Kyouya and Erika are truly meant for each other.