Izumi and Ryouma met as children whilst on the set of a Happy Wedding commercial. Ryouma falls in love with Izumi, under the belief that he was a girl, and spent the next 10 years desperately trying to reunite with "her". When they meet again, they are on set for the sequal to the Happy Wedding commercial that he personally arranged in order to meet back up. It is here that Ryouma is finally convinced of his love for Izumi, confessing his feelings for "her". However, Shougo reveals to a horrified Ryouma that the girl he has been in love with for the past 10 years is actually male. Ryouma is left completely shocked, furious with and yelling at Izumi for "lying" to him before he storms off.
Later, Ryouma heads to the Sena household, still in shock that Izumi is actually a boy. He wants to see Izumi one last time in an attempt to get rid of his feelings once and for all by seeing him completely as a man. However, once he forces Izumi to strip for him, he finds himself attracted to him and begins to further force himself onto Izumi. However, Shougo arrives and stops Ryouma.
After that, things are awkward between Izumi and Ryouma. Izumi is upset over the whole experience, both because of how forceful Ryouma was yet finding that he enjoyed being touched by Ryouma. Meanwhile, Ryouma forces himself to come to terms with the fact that, regardless of gender, he is still very much in love with Izumi. He apologizes to Izumi and confesses his love to him, and the two begin a new relationship as "texting buddies". This continues until Ryouma compliments Izumi's manga and offers to help him finish it before the competition deadline. Their genuine friendship begins there.
They remain friends for a short while, but as they become closer and closer, it becomes clear they are developing a mutual attraction to one another. Izumi finds himself falling for Ryouma and, after suddenly being hit with this realization runs to Ryouma's apartment and kisses him. Izumi tells Ryouma that he wants to have sex with him, which Ryouma eventually agrees to. The next morning, where Izumi is recuperating from his first time having sex, Ryouma asks Izumi if he had asked for sex looking for comfort. Izumi quickly declines, sitting up and regretting it immediately when he remembers the pain he's in. A moment later, he recovers and realises he never told Ryouma the reason he came. He tells Ryouma that he loves him, which results in Ryouma crying and returning the declaration of love. That marks the beginning of their relationship.
Their first date is at a theme park. Izumi secretly asks Rei to have Shougo come, as he is afraid of being alone with Ryouma after the painful sex they had before. Despite being excited for his first official date with Izumi, Ryouma finds that he's forced to spend more time with Shougo. The date continues for a while, before Ryouma manages to lose Shougo, and while Rei is away getting snacks, Ryouma takes his chance and runs away with Izumi. However, Izumi stops. He is hesitant to leave with Ryouma being he's knows Ryouma will "try something dirty". He admits that it was extremely painful when they had sex for the first time, which causes an embarrassed Ryouma's mood to deflate as he leaves. That's not the end however, as Ryouma goes home to study gay sex and the appropriate preparations for it. Meanwhile, Izumi and Rei have a conversation about it, in which Rei suggests that Izumi tops, so that way he won't feel pain. Izumi likes that idea, and heads over to Ryouma's apartment to apologize. They both end up apologizing in sync, which causes them to laugh and share a kiss. Despite Izumi being dead set on topping, Ryouma insists that he's studied enough so that it won't hurt this time. Reluctantly, Izumi agrees to bottom and they have sex for the second time. Afterwards, Izumi appears to be feeling blissful and very content, and praises Ryouma for being so good. Ryouma is relieved that Izumi enjoyed it, and Izumi pecks Ryouma on the lips and asks if he wants to go for another round. That marks the beginning of their sex life.
They continue steadily for a while, their only obstacle being when Ryouma was hypnotized by a director and briefly forgets Izumi. His memory comes back to him when he is smacked on the head by Shougo, who then claims it was intentional. After that, the two continue dating happily.
During a drive home, Ryouma asks Izumi to move in with him. This takes Izumi by surprise, and asks if he means as roommates. Ryouma says no, and explains that he would like to live together as a married couple. Izumi is concerned about how Rei and Shougo will react, but Ryouma simply tells him not to worry and that he can think about it. Shortly after, Rei discovers that Izumi's stalker has discovered where Izumi lives, and begs Ryouma to let Izumi stay at his apartment for a while. Both Ryouma and Izumi are excited by this, since they'll be getting a taste of what married life will be like. In all the current chapters, it appears that the two of them are enjoying living together.
The two appear to be becoming more open about their relationship. After being caught kissing by Kuroi, Izumi happily tells him that he and Ryouma are dating and even offers great detail about their relationship, even urges Kuroi to ask questions. This results in Izumi telling Kuroi all about their first kiss and how much sex they have in the relationship. Shortly after, Ryouma tells his boss that he and Izumi are in a serious relationship and is not changing it for anything.
Throughout both the manga and the anime, Ryouma and Shougo hold a mutual dislike towards each other. This began when Ryouma blamed Shougo for stealing his chance at a music career by stealing the spotlight with his own music debut. Also, Ryouma dislikes him since the public often showed great favor towards Shougo since they always vote him for "nicest celebrity" and "top celebrity I want to sleep with", therefore stealing the spotlight away from him further. Shougo personally dislikes him since he walked in on Ryouma forcing himself on Izumi, therefore not even liking him casually touching his younger brother.
It was also revealed in a manga chapter how when Shougo and Ryouma first met, before the latter met back up with Izumi in Love Stage, Ryouma took note of Shougo's unique eye color (noticing how they matched Izumi's and wondered if the two were related) and began staring closely and intently at him. However, Shougo mistook this stare as Ryouma wanting a kiss and gave him a "greeting kiss" despite being public and on television. This, effectively, shocking Ryouma and starting the two's relationship off on a bad note. Despite the two's complicated relationship, Ryouma often refers to him as Shougo-nii-san.
Ryouma holds a great hate towards Jin Kisaragi since the man is one of the reasons as to why Ryouma and Izumi temporarily shunned each other. Despite it seeming like Jin was in love with Ryouma, he proves to be someone who will try to get with anyone he finds attractive. This included Ryouma, Izumi and even Rei. The latter whom he had even wished to take advantage of in an emotionally vulnerable state (after Ryouma and Izumi's cliff-fall accident), until Shougo arrived to comfort Rei. Ryouma also suffered sexual harassment from Jin, though only to the point of "copping a feel" and nothing too serious.