You can read the whole summary of the Back Stage Novel chapters here.
He was sent to study abroad and during the summer vacation he saw Rei masturbating while thinking of his father Seiya. Shougo thought he was really cute and ended sleeping with Rei.
Rei panicked because he slept with the son of Seiya. So Shougo suggested to just keep this a secret. So they both do a pinky promise to keep it a secret. After they made the promise, Shougo suggested to do it again saying that they already done it once, so what does it matter if they do it two or three times more.
When they were having diner Nagisa asked what Shougo wanted to do after graduating. Shougo tells that he would skip some grades and will see what he will do afterwards. Nagisa is disappointed that Shougo won't be entering the entertainment industry.
When his summer vacation ended, Rei brought him to the airport and asked some questions about love and afterwards left, but he couldn't forget about Rei, so he kept coming over when he had a vacation.
But one day Rei came to his room and told Shougo he wanted to end his sexual relationship with him. And so, their relationship ended just as secretly as it had begun, and without even waiting for his Spring Break to end this time, Shougo returned to England.When he was back in England he kept thinking about Rei and finally went back to Japan and found out that Rei moved out of the Sena house. After asking Rei's address, he went to his house. He told Rei he couldn't forget about him and told him he loved Rei. He knew Rei was in love with Seiya, but he really wanted Rei to only look at him and asked him if he could do anything so Rei would look at him. Rei tells him that he can't return his feelings, because he still loves Seiya. Then Shougo suggest if he surpasses his father would Rei then only think about him and not his father. Rei tells Shougo that if he can manage that, he would think about it.
Shougo vowed to set off on a journey of studies to become an even greater singer than his father. Three years after Shougo disappeared, Rei began to hear rumors about a legendary indie band called the CRUSHERZ that had filled the Budoukan Hall to capacity.After giving his concert and when he was about to leave. He hears somebody yelling from the crowd. He sees that the one yelling is Rei. He was tottaly bolted up and didn't know what to do, so he immidiatly turned around and entered the car. In the car he was worried Rei took it the wrong way and he kept asking what he should do now. His friends told Shougo he should call Rei, but Shougo didn't wanted to do that. Shougo was going to seek Rei out himself after he'd made it even bigger! He was going to make his major debut, hit #1 on the Oricon Charts, sell a million albums and then find Rei!! That’s been the whole point of keeping his name and face a secret this whole time!
The next day he sees Rei at their studio and they both leave to a cafe. There Rei asks him to join SenaPro, but he can't join the label of his rival in love, Seiya. Then he leaves and a couple minutes later he gets a mail from Rei saying he has something important to tell him and if he could come tonight.
Shougo shows up around 1 AM that evening. When he inquires as to what this important thing is Rei called him here to discuss, Rei confesses he lied. He tells Shougo that he still loves Seiya and even if he becomes a milion-album artist his feelings won't change and even told him about the plan to let Shougo enter the entertainment world. Shougo tells him he always understood, deep down–that people’s feelings don’t change so easily. That there was no way Rei would love me if he surpassed his father, but he still thinks he wants to try his best anyways, to try and make Rei fall for him. Just because people’s feelings don’t change that easily–it doesn’t mean Rei's never will. Shougo accepts to join SenaPro saying the can't call himself a man if he can't grant the wish of the person he loves. And afterwards they sleep together. Several days later, it’s formally announced that the Crusherz will sign with SenaPro.
At work Shougo keeps whining and moaning the whole time about ~oh Rei’s so busy we can’t be lovey-dovey~ and ~I’m Rei-deprived!~ and because of that they haven’t been able to get any songs written, so they decided to just kidnap Rei and drag you abroad. Shougo has also been realizing Rei is pushing himself too much and decides to take him to New York where he plans to suprise him with a trip to a deserted island where they can both relax.
~At New York~
Given that they’re there to work, they find themselves mostly stuck in the studio recording all day. They’re sharing a room on this trip, but while Rei had at first been on his guard, in reality Shougo’s been in the studio til late at night every day and collapses into bed as soon as he gets in. He works really fast, because he really wanted to spend time with Rei. He was working his ass of, going on about ‘I’m definitely gonna spend some time alone with Rei!’ Shougo continues to give great work for the rest of their time there and they wind up wrapping up the recording a full day early–meaning there’s still time for some sight-seeing!
That's when Shougo decides to take Rei to the deserted island.
~At the Island~
After swimming and playing in the water and exploring the island a bit they get tired and decide to rest. Just as the sun starts to move on towards the horizon, they collapse onto a beach mat on the sand to catch their breath, exhausted from the day. Shougo tells him he intended to bring Rei here all along, because he really looked tired the past few months and it seemed like he could shake off that fatigue in a place like this.
~Back in New York~
The next day, the group return to John F. Kennedy airport to return to Japan. Shougo realizes after they’ve already passed through security and immigration, though, that he completely forgot to buy Izumi a souvenir.
At this time, Shougo returns, having successfully bought some chocolates that Izumi is fond of, and Rei immediately turns a glare on him. Shougo’s dumbfounded–what’s with this? Rei just reminds him that he had a good vacation so he should be perfectly set to get back to work now.
Shougo was called by Rei who asked him to help to convince Izumi to play in the commercial. Although he was now in London recording a new song he couldn't say no to Rei and he couldn't write a new lyrics, so he took the first plane to Japan. Of course Rei didn't know Shougo was in Japan.
The next day Shougo and Rei meet up in a luxurious hotel. When Rei arrived Shougo gave him a kiss and actually wanted more, but Rei stopped him. Shougo knew that Rei wasn't really in love with him, but Shougo loved Rei too much to give up. Shougo told Rei what he would do to convince Izumi to play in the commercial. Suddenly Rei’s cell phone began buzzing in his pocket. Rei pulled it out and saw from the display that it was a call from the manager of Shougo’s band, the CRUSHERZ. The manager told him that Shougo was actually in London recording a new song and yesterday after Rei called, Shougo took the first plane to Japan.
Rei got mad at him and Rei told him to go and persuade Izumi immediately and then go back to London and Rei will return the favor once Shougo is back in Japan again. The next day Shougo went to Izumi and convinced him and afterwards called Rei that he managed to convince him. Then he went to the airplane, but the weather got so bad that they bumped his flight to a later time, so Shougo went to Rei's house where he could stay. The next day Shougo thought of a new lyrics and went to London again.
-Some Time Skip-
It’s been a few years now since the Crusherz made their debut, and they’ve got their own personal manager now in Sotomura, with Rei handling the affairs of SenaPro in general and being less pressured with the band’s activities. But only just recently, it was decided that the last non-showbiz member of the Sena family, Sena Izumi, would make his debut into the entertainment world.
Shougo’s bothering Rei at the office. He’s already made sure they’re the only ones in the office, and therefore he’s taking advantage of their privacy to make a nuisance of himself. There they decide to meet up again after work at Rei's place and Shougo leaves.
But later on the day Shougo suddenly got a message saying if they could meet up another time since Rei had too much work right now. Shougo understood it and they decided to meet another time. But there was one thing bothering Shougo. The message started with "uhh.." Whenever Rei had something he didn’t want to say, he always tack on "uhh..." to his words.
Shougo got worried, and came by to make sure Rei got home okay. Then he saw some weirdo messing around with Rei's mailbox, but when Shougo called out to him, he took off running. Shougo caught up with him, though, and he spilled everything. He told him about a man who should now be in Rei's house and who wants to rape Rei.
Shougo quikly runs to Rei's place where he hears another man voice. First he waits a bit, but at hearing his name called by Rei he bargs into the room, Shougo flashes Rei a quick smile to put him at ease, then glares at Mitsudzuka. “I’ve got everything on video–if you lay another finger on my Rei, you won’t get off easy. I’ll use all the power at my disposal to end you.”
Tossing back a final few choice words, the guy scrambles from the room, fleeing the scene along with the stalker Shougo had caught. Shougo makes sure to lock the door tight after they leave before rushing to Rei’s side. There Rei asks Shougo to kiss him. Shougo is kind of confussed, but he does as Rei says and afterwards they have sex.
Shougo sees that Rei is smiling and asks what's wrong. Then Rei tells him: “…Seems I apparently actually love you quite a bit.” Shougo was shocked and asked Rei to say it again, but after Rei said those quiet words he passed out.
-Couple months later-
Shougo's birthday is coming up, but Rei tells him he has a concert in Osaka on his birthday. Shougo is still disappointed–he’d really been looking forward to spending his birthday with Rei and tries to wheedle his way out, or at least to have Rei let him get there the day of so they can spend some time alone together before. Rei manages to cajole him into being a good boy and traveling with his companions by promising to celebrate good and hard after the concert. Shougo gives in…but not before talking Rei into a bit of pre-birthday celebrating which proves quite pleasurable for the both of them.
It’s finally the day before Shougo’s birthday. Shougo and his gang are set to arrive on the next-to-last bullet train, so they head to the inn first, where they’ve reserved rooms at an onsen.
A short while later, Shougo and the others arrive safely in Osaka and make their way to the inn–where the surprise goes off without a hitch. Shougo is thrilled, and a cake is quickly wheeled in and the lights doused as everyone sings Happy Birthday.
Shougo blows out his candles and receives congratulations from all around (and is really confused as to why on earth Ryouma is there too), and the party gets going–there’s karaoke and chatting and, of course, booze.
Rei takes a moment for a toilet break, and as he’s heading back to the main room, he finds Shougo standing in the hall, waiting for him.
Shougo suggests they go for a walk in the garden for a few minutes. Just then, though, Haru and Linpei show up, waving a card key for their hotel room–they CRUSHERZ were supposed to room together, but the pair let Shougo and Rei know that they’re going to just bunk with Sotomura-san…so Shougo can have the room all to himself. Shougo drops his voice and suggests they meet up in his room later, and Rei agrees after a moment’s silence.There they 'celebrate' Shougo's birthday together~