Izuku would likely become more cynical. Stain's views are quite extreme, and being trained by him would expose Izuku to those ideas. He might start to question the motives of other heroes more often.
His fighting style would change drastically. Stain is a very different kind of fighter compared to what Izuku has learned so far. Izuku would become more stealthy and efficient in combat. He might also develop a more ruthless streak when it comes to taking down villains. However, at his core, Izuku's kind heart would still remain, but he would be more conflicted between the traditional hero values and the new ones he's learning from Stain. He could end up being a hero who is more of an outlier, not fully fitting into the typical hero mold but still striving to do good in his own way. And this would make his relationships with his classmates and other heroes more complex as they might not fully understand his new perspective.
Naruto would probably become more serious. Shisui was a serious - minded ninja. His training might make Naruto less impulsive and more calculated in his actions.
Naruto's character would likely become more cynical. Orochimaru's ways are full of deceit and self - gain, and Naruto, being around that, would start to question the ideals he once held. His usually bright and optimistic personality might be clouded with a sense of distrust and a more cut - throat attitude.
Naruto would become more cunning. Orochimaru is a very sly character, and his training would likely rub off on Naruto. Naruto would no longer be as straightforward in his battles and interactions. He might start to think several steps ahead, like Orochimaru does.
Naruto would likely become more secretive. Since Orochimaru's training methods are often not in line with Konoha's values, Naruto would have to hide what he's learning. He might also become more distrustful of others as Orochimaru could instill in him the idea that everyone is out to get him for his newfound power.
He'd probably become more adaptable. Naruto is already flexible in combat, but RWBY's diverse combat styles would force him to be even more so. His personality might also become more open - minded as he experiences the different cultures and ways of thinking in the RWBY world.
If Naruto is trained by Deathstroke in fanfiction, his character would change in multiple ways. For one, he might lose some of his naivete. Deathstroke's world is a harsh one, and Naruto would have to adapt. He could become more calculating and less likely to jump into a fight without thinking. His relationships with his friends might also change. He might start to keep them at a bit of a distance, not wanting to put them in danger as a result of his new, more dangerous fighting style.
Naruto's character would become more serious. Sephiroth's training is no joke and Naruto would have to focus more. He might lose some of his naivete and become more worldly.
Naruto's character could change in several ways if trained by Kisame in fanfiction. Firstly, his confidence might take on a different form. Instead of the boisterous and sometimes over - the - top confidence he has, it could become a more subdued and internalized self - assurance. His relationships with his friends in Konoha might also be affected. He might feel a bit of an outsider now that he has been trained by an enemy. Emotionally, he could become more guarded, as Kisame is not one to show much emotion. And in terms of his fighting style, he might adopt a more stealthy and patient approach, which is very different from his usual in - your - face style.
In the fanfiction, perhaps Stain trains Izuku because he believes Izuku has the purity of heart to use power in a way that Stain deems correct. Stain could teach Izuku how to analyze opponents better, which would enhance Izuku's ability to strategize in battles. Izuku might then start to question the current hero ranking system during his training. He could grow to be a hero who is more focused on the essence of heroism rather than just fame and recognition, which would make him stand out among his peers and also cause some friction with the more traditional heroes and hero agencies.
His confidence might become more of an overbearing arrogance. Since Acnologia is so powerful, Natsu could start to think he's invincible after being trained by him. He might become more solitary too, as Acnologia is often depicted as a lone figure. Natsu might start to distance himself from his Fairy Tail friends because he feels he's on a different level now.