Well, he would probably feel disgusted by Kei, which in turn would make him angry at himself for feeling that way about his own brother. He would feel frustrated because he thought he knew his twin, but it turned out he knew nothing. He would feel like suddenly the ground beneath him had been turned-up and his family life shattered. It would be a massive internal conflict of not knowing whether to feel rage at himself or rage at Kei. He didnt want to hate his brother or be angry at his brother but he had no idea how to react to the complete 'sranger' he had just seen that was meant to be his twin. He was confused, terribley confused. Anyone in his shoes would be.
it sucks the story was discontinued, but, with the plot knowledge we were given and after reading the story 3 times, i think i can fill in a couple gaps for everyone as devastated as me :').
Why Roku Jumped With Kei:
Considering Roku's feelings towards Kei is crucial;
yes, he didnt love kei in the way that kei loved him BUT Roku still cared deeply for him. His anger/anguish after seeing the 'true' kei (as kei calls it) wasnt based on the fact that he was just purely angry and disgusted with Kei. Rather, he had a form of identity crisis among other things. He called himself kei's brother and yet in a matter of seconds he felt he knew nothing about the boy staring straight at him. He probably felt guilt for not being there for him, guilt for not paying attention to the red flags. Anger at both himself and kei because things have ended up this way. Seeing his twin in such a state sent him into shock.
In the split seconds leading to Kei making his final jump, Roku couldnt even think straight. There he was, standing 5 meters from his twin that threatened to drop from the building any moment. Someone who had cried just prior to that about his regretful and empty outlook on life;
why were they separated?
why were they so completely different?
they should have been just one entity.
That was all Kei had ever wished. That was the only thing Roku could give him in his last moments. So without any further thought, he jumped along side him.
After the attempted suicide and how Roku's 'partner' came into play:
After the death of Kei, Roku finally understood that feeling of emptiness. He had lost a half of himself that he didnt even understand anymore. He wanted to fill that void, leading to very similar methods to Kei. The guy probably came into Rokus life as the typical form of 'tough friend' that Roku could vent to about his issues after coming back to school, and he also could conveniently serve as something to fill that 'void'. We learnt that Roku went through lots of therapy and hospital visits for sporadic and harmful behaviour towards himself so maybe there is a link there? Nothing kind, just harsh and straight forward. Putting himself in Keis position probably comforts roku in a messed up way.
The use and significance (if any) of Ichinose's character:
I dont think Ichinose serves as anything but a medium in which the story can be told. In a way, looking at it from both an insider, and outsider view gives the story more depth. He'd probably learn of this all, get flustered, say "i-im sorry.. i didnt mean to-" and have Roku reply with something like "no it fine. it was my own choice to tell you" or something. Ichinose would probably feel extreme fruitless guilt.
Really, I can only see the story ending 1 of 3 ways:
1. (most likely) Roku ends up killing himself to join Kei (this has already been foreshadowed by him rejecting hospital visits, going to the roof, smoking, loosing emotion--> strong links between his behaviour now to what Kei's was). This would be a tragic ending, but plot fulfilling and it would make the most sense.
2. Life literally just goes on --> not as sad, but doesnt tie ends together very well. Least likely
3. Ichinose makes it a mission to somehow help Roku. This one could go down multiple pathways that i cant fit in here to discuss but im sure you could think of a few potential ways.
now lets all cry together :')