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When Eraqus died, he gave his heart to Terra.
It is revealed in Blank Points, the secret ending of Birth By Sleep; check it out.
That's what allowed Terra to fight back instead of being consumed by Xehanort's darkness.
Eraqus heart was fused with Terra's heart which allowed Terra's heart to become stronger and withstand Xehanort's darkness.
Xehanort's heart and Terra's heart wasn't fused yet when we fought "Terranort" in BBS. When Xehanort foolishly unlocked his keyblade I believe that's when Terra and Xehanort's heart fused together to make Apprentice Xehanort. A hybrid of Young Master Xehanort and Terra's face/built.
kairi's heart was fused within sora's heart like eraqus' was with terra
Now that we know Nobodies can develop hearts, is it fair to think that Xemnas is AX's nobody harboring Terra's heart and Ansem SoD was MX's heart?
Depends on many things.
- There must be no "terranort" among the 13
- Xemnas himself ignores he grew back Terra's heart and acts following the memories of MX instead
Xemnas seems a little bit too evil to Terra ignoring himself... Or maybe it's a "xehanortified" Terra, which would be plausible.
Also, Riku said that whenever he met Ansem SoD, he remembers Terra. Meaning Ansem SoD probably had some of Terra's heart and left some inside of Riku.
I guess if that's the case, Aqua could bring the Terra side out of him.
Nomura while talking about Terra and Xehanort said that who has what memories and heart, and how they control it, would be a key component to the story in future games.
So anything can happen really, but there's more to it than "MX MX MX MX MX MX" obviously.