If Harry becomes an Apostle in fanfiction, his sense of self - identity could be completely shaken. He has always been defined by his wizarding heritage, but now as an Apostle, he has to grapple with a new set of values and powers. This could make him more introspective and perhaps a bit more of an outcast, even from his old friends at Hogwarts. He may also become more decisive, as Apostles often have to make quick and important decisions in the face of danger.
Well, Harry Potter's character would likely experience a significant transformation. Firstly, his loyalty might shift or expand. In the wizarding world, he was loyal to his friends and Hogwarts. As an Apostle, he may develop loyalty towards a new cause or group. His confidence could also take a hit initially as he has to learn the ropes of being an Apostle. He'd be out of his comfort zone, having to learn new powers and ways of life. However, as he progresses, his confidence might return in a different form. He may also become more secretive as he tries to hide his new identity from those in the wizarding world who may not understand or accept it.