At the beginning of their relationship, Kazune deemed Karin as annoying, noisy, and a crybaby, while Karin who at first, thought that Kazune looked like an ideal prince (someone who was perfect for her). She soon thought that Kazune was a bad tempered, blunt, and impatient boy. Soon, they began to trust each other, relying on each others strengths in difficult situations.
Kazune is extremely protective and possessive of Karin. In the manga Kon Kon Kokon, (where he appeared as a minor character) he stated that he was looking for someone, and most fans generally agreed that the "someone" was Karin.
Kazune Kujyou's relationship with Karin is probably the most recognized one. This is because at the beginning of the series, they kept fighting which obviously showed their close relationship. Many hints have been shown of their relationship throughout the entire series. In the very first episode, Karin punches Kazune because he was being sexist.During Episode 5, Kazune helped Karin study for the next test, so that she wouldn't get kicked out of school.In Episode 7, Karin gives her costume to Himeka when she is too embarrassed to wear a sumo suit. This implies that he cares for her. Also, in Episode 10, Karin comforts Kazune when he worries that he is useless. They show a close relationship in that Episode that Karin is able to transform with Kazune's ring, and Kazune is able to transform with Karin's ring.
In Episode 12, Karin overhears Kazune confessing his love for her to Michiru Nishikiori. During Episode 14, Karin realizes her feelings for Kazune.
Episode 15, Kirio Karasuma falls in love with Karin in a blonde wig. Kazune and Karin overcome obstacles to get the ring that he thought was for Karin with a blonde wig, and when they fall into a pit, they almost kiss. This shows that Karin and Kazune care a lot for each other.
In Episode 19, when they act out a play, Kazune and Karin basically confess their love (her saying she still loves him even if they have the same father; he saying he shares the same feelings). Also, Karin cared a lot about Kazune's welfare when he fell ill during the play.
In Episode 20, Kazune eats some Karin's cooking, (which is really bad) and gets drunk. When Kazune is drunk, he acts perverted towards Karin, and so, he kisses her (their first kiss together) after implying that he was jealous of Micchi for getting all the attention from Karin.
During Episode 21, Karin and Kazune are embarrassed by their kiss. Also, when Kazune gets defeated in battle, Karin takes care of him. In the manga, she takes her clothes off for Kazune to preserve body heat.
In the last episode, Episode 26, Karin remembers her past. She was the wife of Kazuto Kujyou, and was aged back to protect her from harm. And, since Kazune is the copy of Kazuto, she is technically his wife. All in all, Kazune's relationship with Karin is a faithful one. They love each other very much, as you can tell, if you know that they kissed more than five times in the entire series.
Also, in Kamichama Karin Chu, they still live together, but not officially as a couple, even though they act like one openly. They meet Suzune, their future son, whom they love and adore. The discovery of his future with Karin and the new chaos of the Karasumas makes Kazune more committed to his family and protecting Karin and Suzune no matter what. Jin is Kazune's new love rival, and Kazune is constantly jealous of him. Seeing that Kazune gets drunk easily, acts perverted only toward Karin, and shows his true feelings when he is drunk. In the end, Kazune and Karin get married. In the bonus chapter of Chu, where Kazune is preparing to enroll in an all boys school and Karin is trying to go with him, she asks him to tell her that he loves her. He tells her that he loves her more than anyone. In the manga, he was seen to be tempting to have sex with Karin after stating that he loves her in the final Chapter in Kamichama Karin Chu♥