He would likely become more secretive. Since ANBU operates in the shadows, Naruto would have to keep a lot of things to himself, which is a big change from his usually open - minded self. Also, the death of Asuma could make him more ruthless in combat, as he realizes the harsh reality of the ninja world. He may still hold onto his core values of protecting his friends and the village, but his methods could be more extreme in the ANBU.
In this fanfic, Naruto's character would experience a significant transformation. The death of Asuma is a major blow to him, and joining the ANBU would expose him to a whole new level of danger and secrecy. He might start to question some of his previous beliefs. For example, he may struggle with the idea of having to kill rather than just defeat opponents as he did before. His relationships with his friends could also change. He might distance himself from them to protect them from the dangerous world of ANBU. At the same time, he could gain a new sense of purpose, to avenge Asuma and protect the village in a more covert way, which would shape his character into a more complex and mature version of himself.
Well, one possible plot development could be Naruto joining the ANBU to seek revenge for Asuma's death. He might use the ANBU's resources and training to become stronger and hunt down those responsible.
He'd become more reserved. The ANBU environment is secretive and doesn't encourage open emotions. Naruto, who is usually so exuberant, would have to hold back. For example, he couldn't go around shouting about his dreams as freely as before.
His character could change in several ways. Firstly, he'd likely become more stealthy. In the ANBU, being able to move without being detected is crucial. Also, he might become more cynical. The ANBU deals with some dark stuff, and that could make Naruto question the world around him more. He may still hold onto his core values of protecting his friends and the village, but he'd view things from a more jaded perspective. He'd also have to be more calculating in his actions, thinking several steps ahead like the other ANBU members.
He would likely become more withdrawn. Naruto is usually so open and exuberant, but heartbreak could make him close off. In ANBU, he might be quieter, more focused on the mission at hand. He'd still have his determination, but it would be a more solemn determination. His sense of humor might also disappear for a while as he copes with his pain.
His relationship with Sasuke would be interesting. Sasuke might be initially suspicious of Naruto's decision to join such a secretive group. But Naruto could convince Sasuke that he's doing it for the greater good of the village and the ninja world, and they could end up working together on some behind - the - scenes missions.
Naruto may become more serious and stealthy. ANBU work requires a certain level of secrecy and discipline, which could tone down his usual exuberant personality.
His relationship with the Leaf Village would become more personal. He'd be part of the Sarutobi clan in a sense. His bond with Shikamaru and Choji would strengthen as they are Asuma's students. Naruto would also have a new father - figure relationship, which would change how he views authority.
He would be highly skilled in combat from a young age. The ANBU Commander would train him rigorously, so Naruto could be a prodigy in various ninja arts. His fighting style might be more calculated and less reliant on brute force and the Nine - Tails' power. He would have a better understanding of strategy and tactics.
Naruto's character would likely become more reserved at first. He'd be constantly fighting his own demons while trying to fit into the ANBU. As he goes on missions, he'd start to learn more about the darker side of the ninja world. This new knowledge might make him more cynical initially, but over time, he could use it to grow stronger. His depression might also fuel his determination to prove himself in ANBU, leading to him becoming a more complex and mature character.
His character would develop in multiple ways. Firstly, the ANBU's harsh training and dangerous missions would toughen him up even more. Secondly, being around other ANBU members, some of whom might be cold and calculating, could make him more cynical or, on the other hand, make him strive to bring more warmth to the group. Also, his brokenness might make him more introspective, leading to a deeper understanding of his own power and purpose within the ANBU.