He would likely become more cautious. After experiencing such tragedies in his first 'life', he would think twice before making hasty decisions. This could make him a more calculating and less impulsive leader.
Robb Starks would probably become more politically astute. He would understand the importance of every decision and how it can have far - reaching consequences. He might also become more empathetic, having seen the pain and destruction his actions (or inactions) caused in the past. He could use this new - found understanding to build better relationships with his vassals and allies.
He might change the outcome of some crucial events. For example, he could prevent the Red Wedding from happening, saving his family and allies. This would completely rewrite the story's future, leading to a different power structure in Westeros.
Noctis' character could develop a greater sense of responsibility. Since he has seen the consequences of his actions in the future, he would be more cautious and calculated in his decisions when he goes back in time. He might also become more self - reliant as he realizes he can't always rely on the same people or circumstances. For example, if he lost someone important in the future due to his actions, he would be more independent in his choices to prevent that loss again.
She might become more empathetic. Meeting people from the past, especially those who were suffering or in difficult situations, could make Serena more understanding. She could see the world from a different perspective and this would change how she interacts with others in her own time. For example, if she sees how people in the past fought for equality, she may be more involved in similar causes in her present.
She could become more independent. In 1918, she would be in a strange time without her usual support system. This would force her to rely on herself more, making her stronger.
She might become more confident in her earlier years. In the original story, Arya faced many hardships that made her grow in a certain way. But if she goes back in time, having the knowledge of future events, she could be bolder in her actions and decisions from a young age. For example, she might be more assertive in dealing with Sansa when they were kids, and not let Sansa's girly ways get to her as much.
In this fanfic scenario, Anakin's character would develop with a greater sense of purpose. He would no longer be a victim of fate. He could take control of his destiny. He would be more self - assured, knowing what lies ahead. His relationships with other characters would also be different. He would be more of a leader, guiding others based on his future knowledge. This would transform him into a more positive and influential figure in the Star Wars universe.
Naruto's relationships would be different. He might be more of a mentor figure to his younger self or his friends. Since he has the power of the Juubi, he could offer a different kind of protection and guidance. He could also have a different view on his enemies, as he might be more inclined to try to change them rather than just defeat them, using the Juubi's power as a tool for persuasion.
His character could become more humble. Going back in time and seeing the past versions of people and events might make him realize how much he has changed over the years. He may also become more cautious. Knowing the consequences of certain actions from his original timeline, he'll be more careful with his decisions in the past. For example, he might think twice before revealing his identity as Iron Man too soon.
Sansa would be more assertive in her political moves. She has seen the consequences of inaction and bad decisions in the future. So, she would be quicker to make decisions that benefit her family and herself. For example, she might be more involved in military strategies to protect the North.
Natsu's character could develop a greater sense of responsibility. Going back in time, he realizes the importance of his actions on a whole new level. He might become more cautious in his decisions, as he knows the consequences they can have in the future.