except the author's random decision to retcon the series and basically restart the characters outside of the MC's tribe after 50 chapters or something
If you exclude the fact that he had always been obedient and been a 'model tribesman', and talk about his inclusion in that time when the children stole the eggs of that precious/divine bird, that still doesn't justify such a change in personality. Not only was he not the main perpetrator/mastermind behind the trip (mind you he was only 3 or 4 years old at the time), the experience should have showed him the dangers of the world and how inexperienced/ignorant he actually is. Other than that one experience that could have influenced his personality, he hasn't come into any sort of influence of trigger that could have caused his personality change, yet after a few years of (seemingly peaceful) training in the tribe, he leaves and becomes greedy over all sorts of treasures and precious resources, even to the point where he baits people into going towards him so that he can collect their treasures and resources, and then even blackmail/ransom them from their clans. Like wtf?
Granted, the people who he ends up baiting were aiming for his own treasures first, so I can somewhat understand him wanting to get them back, and even with interest (ignoring how he was even able to come up with a plan like that), but when he later joins a new school and then meets with one of the school's beauties, he starts being lewd towards her. Like what? When did this 9 or 10 year old suddenly know how to be lewd? That's just ridiculous.
Then, some time after that, when he had managed to take some spoils (in the form of resources), he randomly thinks about his tribe and how he could share his spoils with them. While that's not bad, again, it's just so sudden. If, at the start of his personality shift, it was made clear that his goal had been to benefit the tribespeople, then his greedy personality could be somewhat explained, but it never happened. He had basically never even mentioned his tribe until that moment, a significant amount of time after he had left, and a significant amount of time after his change in personality. It's just ridiculous.