How then, do you explain the phenomenon of those that have had a complete personality change due to a severe head injury?
I of course do not personally know who you are talking about.
In the case of obviously negative changes > I think it could be connected with the person's real character. People can change in very negative ways, even without physically damaging things happening to them.
The one you marry can turn out to have a very different personality, than what you supposed you were seeing before getting pronounced. On we could go. Therefore, someone having the same character can show extremely different personalities, even without any injuries. The sweetie in public can be a tyrant at home, right?
But in general > there are people who never have committed themselves to being gentle and humble and unconditionally loving, like Jesus. And so their personalities are weak . . . deeply weak . . . and unstable so they can go through temporary or long-term changes more or less severe.
You have seen how a person's personality can change quite a lot, simply if he or she does not get his or her own way. So, in the case of a very negative change . . . maybe it is something like this?
But what if a kind and caring person takes a hard hit? I would say the person's new personality would be kind and caring, even if greatly different . . . if the person's character was genuine and the person was not putting on an act. And the reason for the very different outwardly-acting personality could be simply because the person's memory has been altered so the person is not remembering his or her ways from before the accident. May be, for instance, the person might become more talkative because he or she forgot that others in the family were the ones who usually did most of the talking while they are together; yet, the injured one is still kind and caring.
But, like I say, I don't know each person. But I understand that when God makes your character His way, it stays this way.