At first, Riku, but, he's managed to leave the past behind him so...
Roxas. Most definitely Roxas.
How would you feel if you were told that "...you were never suppose to exist?"
Honestly, everyone's been through things but Roxas has it all.
He's been constantly used for the sake of retrieving Sora's memories and had a fake "normal" life created so that he can feel "happy".
He's had pain { physically and mentally } about someone he doesn't even know { Sora }, at first, and finds out he is not, well, himself.
This young kid, literally finds out that he has no heart and no emotions and undergoes trauma and even close to depression because he was never suppose to exist.
When Roxas is told to "go back to Sora" in order to complete him, he denies it, cries, and wishes to just have a normal life like Hayner, Pence and Olette.
Roxas also had to see his two best friends go. He saw Xion dissapear and then lost his memories to Axel, then, disappeared completely before Axel could even see him again.
I'm serious like honestly, he's the only one who's never accepted his fate and it's just so horrible to even think of what this poor boy's been through.
Roxas just wants to be his own person.
Sora even agrees too saying that Roxas deserved a life better than him.