His character would likely become more conflicted. He has always been a positive and determined person, but being around Orochimaru would expose him to a lot of darkness. He might start to question his own beliefs and values. For example, he may become more ruthless in battle as he adopts some of Orochimaru's combat styles.
Naruto's character would likely become more conflicted. He has always been on the side of good, and joining Orochimaru, even if for a good reason, would put him in a morally gray area. His view of power might change as he is exposed to Orochimaru's unorthodox ways of obtaining it.
If he joins an organization, Naruto might become more strategic. Since organizations usually have a set of goals and plans, he'd need to think more about the overall mission rather than just charging in like he used to. His leadership skills could also improve as he might be given tasks to lead a small group within the organization.
Naruto's character would become more complex. His initial naivete about the world's darker aspects would quickly disappear. He'd become more cynical as he witnesses the actions of the Dark Brotherhood. His relationships with his old friends in Konoha would also be strained as they might not understand his new allegiance.
His character could develop in a darker direction. The torture and manipulation from Orochimaru's experiments could make him question his values. However, it could also strengthen his resolve to protect his friends and the village. He may start to see the world in a more complex way, understanding that there are greater evils out there. And his newfound powers from the experiments could give him a sense of responsibility that he didn't have before.
Naruto's development would be a journey of self - discovery. As Orochimaru's apprentice, he would be exposed to different forms of power and knowledge. He might initially be disgusted by Orochimaru's methods but then realize that he can use some of it for good. His social skills would change too. He would be more adept at dealing with enemies and those with different motives. His sense of humor might even change, becoming darker to fit in with Orochimaru's crowd. But deep down, his core values of friendship and protecting the weak would still be there, just masked by his new persona as Orochimaru's apprentice.
Her character could develop in a way that she becomes more independent. In the fanfiction, away from the protection of her usual teammates, she has to fend for herself among Orochimaru's minions. This would force her to rely on her own strength and intelligence. Also, she might develop a darker side. Being around Orochimaru's evil influence, she could struggle with maintaining her positive outlook and might have moments where she is tempted by the power he offers without considering the consequences.
His character could become more complex. He'd have to balance his own values from Konoha with the new ones in the Covenant. This internal struggle would likely make him more mature. For example, he might question his previous beliefs about power and leadership.
His character could become more adaptable. In Kumo, he'd be exposed to different values and ways of life. This would make him more open - minded. For instance, he might start to view the concept of power in a new light, as Kumo has a different emphasis on strength compared to Konoha.
Naruto's character would become more conflicted. His usual bright and positive attitude might be dampened by the darker environment of Sound. He'd start to question his values. For instance, he might be forced to participate in battles or actions that go against his beliefs in Konoha.
His character would likely become more diplomatic. Since the Fire Nation has a complex political structure, Naruto would have to learn to navigate it. He might become more calculating in his actions, but still retain his core values of friendship and justice.