How would the characters in Bleach react if Ichigo betrays and joins Aizen in a fanfic?

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.

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.

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.

At the start, Ichigo would be defiant. He would not show any fear in front of Aizen. He might taunt Aizen, trying to get him to reveal his true intentions. As time goes on, he would start to feel a sense of isolation, but this would only strengthen his resolve. He would use his inner Hollow powers more freely, as he has no other option. He would also be constantly thinking about ways to turn the tables on Aizen and escape, and would be ready to take on any challenges that come his way during his captivity.

Rukia would probably be very suspicious right away. She knows Ichigo well and would notice the strange changes in his behavior. She might try to confront him and figure out what's wrong.

Rukia would probably be both amazed and a bit worried. Amazed at the new power Ichigo has, but worried about how it might change him. Chad would see it as a natural progression of Ichigo's power and be supportive. Orihime might be a bit scared at first, but then she would see the good in Ichigo's new form and try to encourage him.

Aizen might be initially dismissive, thinking it's just another mortal event. But as he realizes the potential power the baby could possess, he could become envious or see it as a new pawn in his schemes. He might start observing from the shadows, waiting for the right moment to make his move.

Rukia would probably be one of the first to be both shocked and amused. She might tease Ichigo a bit at first but then quickly get serious about helping him. Chad would be very accepting and offer his protection as usual. Uryu might be a bit more skeptical at first, thinking it's some kind of new Hollow trick. Orihime would be very empathetic and try to make Ichigo feel comfortable in his new form, maybe even offer to make some new clothes for him to fit his new body.

Many of the captains would be on the edge of their seats. They'd be closely observing every move Ichigo makes. Ukitake might be silently cheering Ichigo on, hoping that he can succeed where they might have failed in the past against Aizen. And Hitsugaya would probably be analyzing Ichigo's techniques to see how he can improve his own skills.