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.
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 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.
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 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.
The Hogwarts students would be divided. Some would be in disbelief and would still hope that Harry has a good reason for joining. They might secretly support him. Others, especially those who have lost family members to the Death Eaters, would be furious and see Harry as a traitor. Harry's old enemies at Hogwarts, like Draco Malfoy, would also be confused. Draco might initially think that Harry has finally seen the 'right' side, but as he realizes Harry's true intentions (if they are good), he would be conflicted as well since he's been brought up in a Death Eater - influenced family.
The other characters would be shocked. Weiss would probably be the first to try and figure out what's wrong with Ruby. Blake would be worried about Ruby losing her way, and Yang would be furious. Yang might even go on a solo mission to bring Ruby back, not fully trusting the rest of the team to handle it delicately. The rest of the characters like Jaune and Nora would also be in disbelief and would start planning ways to rescue Ruby.
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.
The Order of the Phoenix would see Harry's joining of the Death Eaters as a major setback. They would likely view it as a sign that Voldemort has become even more powerful. Dumbledore, if he were still alive, would be very concerned and would try to understand what led Harry to this decision. He would also be devising a plan to get Harry out of the clutches of the Death Eaters.
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 other characters' reactions could vary widely. His close friends in the fanfic might be in a state of panic, crying and not knowing what to do. Enemies, on the other hand, might see it as an opportunity to take advantage, or they could also be secretly worried depending on the complexity of their relationships. And there could be those characters who are just bystanders, shocked at first and then trying to figure out what happened.