Some characters like Ichigo would be extremely suspicious. Given his past battles with Aizen, he would not trust him easily. Ichigo might keep a close eye on Aizen's every move, waiting for any sign of his old devious ways to resurface. On the other hand, some of the more naive Shinigami might be easily fooled by Aizen's charm. They could see his joining as a great opportunity and welcome him with open arms, not realizing the potential danger.
Characters like Rukia would be conflicted. She has a strong sense of justice and loyalty to the Shinigami. While she would understand the potential benefits of having someone as powerful as Aizen on their side, she would also be haunted by his past actions. She might try to reach out to him, to understand his true motives for joining, but at the same time, she would be prepared to take action if he showed any signs of returning to his old ways. And then there are characters like Kenpachi, who might just see Aizen as a new target to fight, regardless of whether he is now on the Shinigami side or not. He would be eager to test his strength against Aizen and prove himself.
The older and more experienced Shinigami, such as Yamamoto, would be very cautious. They remember Aizen's past treachery and would likely put strict restrictions on him. They would probably assign him tasks that are closely monitored and make sure he doesn't have access to any crucial or dangerous information. The younger Shinigami, who have only heard stories about Aizen, might be divided. Some would be in awe of his power and reputation and be eager to learn from him, while others would be influenced by the warnings of the older Shinigami.
Aizen joining the Shinigami in a fanfic could lead to a complete change in the power dynamics. Since he is a very powerful character, his addition might disrupt the existing balance. He could start training some of the younger Shinigami in his own way, which might be a blend of his devious methods and the traditional Shinigami training. This could create a new generation of Shinigami with a different perspective. Also, his past actions and the knowledge he has gained from his time as an antagonist could be used in unexpected ways. For instance, he might use his understanding of the Hollow world to create a new defense mechanism for the Soul Society against Hollows, but with a hidden agenda that only unfolds later in the story.
The Soul Reapers in the Soul Society would see Ichigo as a major threat. They would likely mobilize their forces to either capture or eliminate him. Captains like Byakuya would be disappointed as he had come to respect Ichigo to some extent, and now would see him as a traitor.
Rukia would be extremely shocked and disappointed. She has always seen Ichigo as a force for good, and seeing him side with Aizen, who is the main antagonist, would be a huge blow to her. She might try to talk Ichigo out of it at first, and if that fails, she could become his enemy.
The Soul Reapers would be very confused at first. They are used to Aizen being a certain way, and Naruto's different personality would be a shock. Ichigo might be curious and see Naruto - Aizen as an ally against the common enemies. Orihime could be one of the first to trust Naruto - Aizen because of his kind - hearted nature that she would sense. And the Espadas would probably be on high alert, not knowing what to expect from this new Aizen.
Well, one possibility is that Aizen somehow convinces Ichigo that his way is the only way to protect his friends. Maybe he shows Ichigo a false future where all his friends die if he doesn't side with Aizen. Ichigo, being desperate to save them, might then join Aizen, thinking he can change things from the inside.
Ichigo would likely be defiant. He's a fighter and would not easily submit to Aizen. He'd probably be constantly looking for a way to escape and would be sizing up Aizen's every move, trying to find a weakness.
The Akatsuki members themselves would have mixed reactions. Some, like Itachi (if he were still alive in the fanfic), might see Naruto as a potential asset but also a threat to their long - term goals. Kisame might be amused by Naruto's presence and see him as a new source of entertainment. Meanwhile, the Leaf Village elders would be outraged. They would see Naruto as a traitor and start devising plans to either bring him back or eliminate him.
The teachers at Hogwarts, like Dumbledore (if he were still alive in the fanfic), would be deeply concerned. Dumbledore always saw great potential in Harry for good. He would probably try to understand what led Harry to this path and would use all his wisdom and magic to try and turn Harry back. The other teachers would also be on high alert, fearing for the safety of the students and the future of the wizarding world.
Well, he might make some new and interesting friends. For example, he could meet other characters with unique powers and personalities. He would also have to learn all the rules and regulations of the Shinigami world, which could be quite a challenge for him as he's used to his own way of doing things. And of course, there would be a lot of training involved, which would help him become an even stronger Shinigami.
The storyline could start with Naruto somehow ending up in Aizen's world. At first, he would be confused and on guard. But as he gets to know Aizen, he might see some potential in him. Naruto would try to change Aizen from the inside. Aizen, on the other hand, might see Naruto as a useful tool or an interesting experiment.