Harry's personality would change in many ways if adopted by the founders in fanfiction. He'd be more mature at a younger age. The founders' influence would make him more aware of his place in the wizarding world from an early stage. He would be less likely to seek validation from his peers like he did at Hogwarts, because he'd be getting it from the founders themselves. His sense of justice would be more in line with the founders' original ideas, which could make him more strict in dealing with dark magic and those who misuse magic. This new personality would shape his relationships, his actions, and his overall role in the wizarding community.
If adopted by the founders, Harry's personality would transform significantly. He would probably be more reserved and studious, as the founders' teachings would focus a great deal on knowledge and the proper use of magic. He might also be less impulsive since he'd be under the strict guidance of the founders, who were very much about discipline and order in magic. Additionally, he could be more authoritative as he would inherit the founders' sense of leadership and responsibility for the wizarding world.
His personality would likely become more self - assured. Growing up with the founders, he'd be constantly exposed to great magic and wisdom, which would boost his confidence. He might also be more independent as he'd be trained in a more solitary way by the founders.