There goes my theory ... But it's possible he only lost his connection to the memories (until he drained them). After all Roxas (and Sora for that matter) never had access to Ven's memories, yet he was still able to use Ven's keyblade.
I wouldn't go as far as saying it can never be undone (the body does perish after all, whereas the heart is eternal), but I agree it's always there; otherwise, the recompletion process couldn't ever take place (as the original body and heart are calling to each other, even after 11 years as seen with Xehanort) ...
But could he grow a heart that fast though?
It's not that I want an explanation at all costs (I already have my own ideas, which we are discussing), I just find it fun debating about these stuff and making theories.
There are only three possibilities I can think of though:
- The nobodies can access the memories in the heart they are connected with (the heartless however cannot, due to darkness taking over or something)
- The memories flow into the nobody from the heartless, but this leads to the question: "where do the memories go then?" The brain? The whole body? Or like you said a part of the heart lingers in the nobody, draining all the memories and (with time and enough stimuli) growing into a new heart (kinda like a liver)
- A combination of both ...
I believe Tinny's explanation is the one that makes the most sense, though there are still other possibilities.
I do realize this will likely never be explained in the series, and it doesn't need to be as it's not even a pivotal point of the story. But it still warrants a discussion I think.
Roxas never lost any connection to Sora.
That he has Sora's memories (after Naminé messed up everything in CoM) is plainly shown in Days when Roxas starts to have dreams and visions about the "boy in red".
However since Roxas' own sense of self has already started to develop he doesn't associate those memories with himself.
That both of them never had access to Ven's memories has to do with the state of Ven's heart. It's injured and inactive and thus sealed away. Kairi's heart for example was active and awake, thus why Sora saw apparitions and sometimes memories of her during KH 1.
Roxas could also only access Ven's Keyblade (and thus dual-wield) after the emotional shock of Xion's death triggered Roxas accidentally accessing Ven's heart and "activate" his Keyblade.
That he had not any "contact" with Sora's memories before was because Sora's own chain of memories was still intact and Sora himself used the memories. It was only after Naminé undid the links of Sora's main chain of memories that some of the links (memories) started to sink into Roxas through his connection with Sora.
From there, Xion started to unwittingly absorb these memories and also to "pull" more loose chain links out of Sora's heart.
That's why it became increasingly difficult for Naminé to restore Sora's memories, as they were "escaping" through his connection with Roxas.
The connection was always there and active (Vexen could create the Twilight Town card in CoM through Sora's connection to Roxas), but there was no memory transfer at all because after Sora was restored by Kairi they co-existed as independent, but connected individuals. Any memory transfer to Roxas afterwards was a result of Naminé unhinging Sora's memories.
How fast a heart grows depends on how many connections a being makes with others, how deep these are and if there are other hearts in proximity.
Roxas was constantly connected to Sora's heart, one of the most powerful around, and had Ven's heart (although damaged and inactive, it is still present) directly inside him.
In CoM, Axel is "subjected" to contact with Sora's heart just for a short time and the development of his "replacement heart" skyrockets as he remarks in one scene "Hey wait, I'm really enjoying this. You guys are something special!" and places a hand on his chest.
These connections already sped up his "forming" of a heart and if you look at Days, you'll notice that Roxas is the Org member who has the most contact and interaction with other beings, not only Axel and Xion or other Org members, but also several other Disney and original characters like Phil, Pete, Tinkerbell, Genie, Pence etc., or, as Ansem the Wise puts it:
Ansem the Wise said:
The heart has always been quick to grow. Each exposure to light, to the natural world, to other people, shapes this most malleable part inside of us.
Sora was the only one able to return to his human form without destroying his Nobody. That is a statement to the love in his heart for other people, and the bonds that tie them together.
Just by being connected to Sora gave Roxas already a huge "starting boost" so to say.
As for how the "normal-case" Nobodies "access" the memories of their original self, since only very strong hearts even can create a Nobody (and of those only the strongest result in a human-shaped Nobody), there has been the theory floating around that the original heart when lost to darkness leaves an "imprint" of sorts in the spot where the heart has originally been and it is there where eventually the transferred memories gather.
Given the proper conditions and "nurturing" as Xemnas put it, that imprint may eventually grow into a new heart.
With Roxas, that imprint however was "empty" due to Sora regaining his humanity and eventually resulted in Roxas developing a unique heart independent from his original, with the information about his appearance being provided by Ven's damaged heart.