Harry's character would develop in many ways if he joined the army in fanfiction. He'd have to learn to trust his new comrades as much as he trusted his Hogwarts friends. His sense of justice would be put to the test in different scenarios. For example, in the wizarding world, he was fighting against pure evil in the form of Voldemort. In the army, the lines between good and bad might be more blurred. This would force him to think more deeply about what he's fighting for and make him a more complex and mature character.
His character would become more disciplined. The Marines' training would shape him further.
His character might become more worldly. At Hogwarts, he was mainly dealing with the wizarding world's problems. But in Shield, he'd be exposed to a whole new set of challenges and cultures, which could make him more adaptable and open - minded.
Harry Potter would probably develop a stronger sense of justice in the context of the BAU. In the wizarding world, justice was often about good versus evil in a very clear - cut way. But in the BAU, the lines can be more blurred. He'd have to learn to understand the complex motives of criminals. His relationships with the BAU team members would also shape him. He'd learn from their different perspectives and become more empathetic towards Muggles and their problems.
His character could become more rebellious. He might start to question the rules of Hogwarts and the wizarding world in general. For instance, if the gang has a disdain for the Ministry of Magic's regulations, Harry could be influenced to think the same way. This would be a big change from his usually rule - following self, at least when it comes to the good rules.
In fanfiction, if Harry Potter joins the army, he'd likely bring his magical skills. He could use spells for things like reconnaissance, making invisible barriers for protection, or even using the Patronus charm to boost morale. His experience fighting Voldemort would give him an edge in combat situations, and his leadership skills developed during his Hogwarts years would make him a valuable asset in military strategy and inspiring his comrades.
He would likely become more hardened. The Grey Wardens face constant danger and hardship, so Harry would have to toughen up. His view of the world would expand as he gets involved in a different kind of fight against evil.
Harry's character often becomes more shrewd and politically aware. In Slytherin, he has to be. He learns to use his wits in different ways than in Gryffindor. For instance, he might become better at reading people's motives and using that to his advantage.
Percy's view of the mortal world would expand. Being in the army, he'd see a different side of mortals - their teamwork, their sense of duty. He might also learn new skills like military strategy that could enhance his demigod fighting abilities. His loyalty might be tested as he has to balance his loyalty to his demigod friends and his new military comrades. And his sense of self - worth could change as he discovers new ways to contribute in a military setting.
His character could become more confident in his charm abilities. After all, the Veela are known for their allure.
If Harry joins the Varia in fanfiction, his character would undergo a substantial transformation. His moral compass might be adjusted as he is exposed to the Varia's different ethical code. He might become more ruthless in combat situations, as the Varia are not as restrained as the wizarding world in terms of using force. His relationships would also change. He would no longer be surrounded by his Hogwarts friends, but by a group of tough - minded individuals. This would force him to communicate and interact in new ways. His identity would shift from being mainly a wizard from Hogwarts to being a member of the Varia, which would influence his self - perception and how he presents himself to the world.