Art by: https://melo-9ba.tumblr.com/
In Kingdom Hearts we already saw that a single person like Sora can have literally many hearts inside of him. Despite those many hearts inhibiting his body that belonged to him in the first place we see our young protagonist fight on all the way, carrying them cautiously inside of him.
With Ventus who was broken apart once, the other half becoming Vanitas and losing his way searching for a safe place in Sora’s heart we have the first candidate that placed himself there.
Another one who this quote was dedicated and meant to is someone who eerily looks like the young man Ventus who placed himself in Sora’s heart as a child, Roxas.
Roxas is the nobody who was born when Sora sacrificed himself and directed the keyblade at the end of Kingdom Hearts I, releasing both his and Ventus heart for a short period of time, therefore Roxas due to Ventus being part of Sora had a similar appearance to the young man, yet Roxas was never supposed to exist in the first place.
And never being meant to exist was something that everyone tried to tell Roxas from the very beginning, “you are a nobody, someone was never supposed to exist yet you exist, but you still have neither a heart nor are you a true person.”
With that Roxas had to live on, slowly learning about the worlds and what it really meant to exist alongside Axel and Xion, giving that life of his, a meaning despite that it didn’t need one in the first place because of his lack of heart.
As Roxas later on lived in a simulated Twilight town in which he – without his own knowledge- made friends with the others and lived on as a regular teenage boy not remembering his life as a nobody nor that he was one in the first place. Yet before he could really live a decent life he was pulled out of it very quickly as he only was meant to help Sora rehabilitate and recover.
In order to do that Roxas needed to vanish and go back where he belonged to in the first place: Sora.
Therefore we often saw Roxas in the other series being envious or angry when people called him Sora, they never referred to him as an own person but simply a puzzle piece or shadow of someone else in which he always countered, “I am me” trying to justify his own existence.
But nobody beside a few people seemed to see that as Roxas finally realized that the lucky person he needed to recover and sacrifice his own existence for was no other than the one he was born for, the one who had a heart and the by-product Roxas split from.
In the end even Roxas when he saw Sora sleeping silently and awaiting to awake at the beginning of Kingdom Hearts II seemed to look reality in the eye and couldn’t hate the person he was always compared to, with his fake life in Twilight Town vanishing in front of him and having no recollection of Axel and Xion, he finally decides his own summer vacation came to an end.
Later on, we see Sora having recollections of Roxas memories and recalls some of the emotions he had towards certain people like Hayner, Pence and Olette as he is still deeply connected to him in one way or another despite the former statement of organization members that a supposed nobody shouldn’t have a heart and therefore no emotions.
After Sora gets to meet and talks to Roxas, Roxas who always asked himself why it was Sora who deserved to be his own person and not himself the pure-hearted Sora uttered the very quote: “You deserve as much as I do to be your own person.”
What Sora is telling Roxas, he does not think of him as a nobody, and he saw with his own eyes how much Axel risked in order for the others to succeed which means for him a nobody can be just as much as a person like anybody else. What he is also telling him is that Roxas is comparing them both with that, he measures their value in a way that hints that only one of them could be a person that should have existed.
Because Roxas once said it could’ve been the other way around, it implied that one of them doesn’t “exist” as their own person, and Sora tries to refute that statement clearly as he speaks of you and I, he includes both of them telling them despite that they are now sharing one body they are still separated beings, both having different lives.
So Sora tells Roxas, it is not only you or I that deserves to be their own person, but just like Sora was his own person from the very beginning Roxas deserves to be one too whether he sees it or not, and despite what others tell him.
For Sora it’s a fact and therefore he tries his best to search and give Roxas the body and existence he deserved in order for others and Roxas himself to be seen as an own person, a separate being from Sora.
Both may have originated from the same source yet it doesn’t mean that they are one and the same or that one is less than the other, because Sora knew that Roxas had people who cherished him too, enclosed in their hearts and therefore when he told Roxas those words he knew he already was an own person despite what he himself thinks.