I really like 'Itachi's Unseen Love'. In this fanfic, Itachi's love for Sasuke is portrayed in a very emotional way. It shows how Itachi sacrificed so much for his younger brother. The writing is very engaging and it makes you feel the depth of their relationship.
The twin brother's character might be developed based on his past. If he had a difficult childhood, it could make him more reserved or cautious. But as he gets to know Naruto and Sakura, he starts to open up. He could start as a loner type, but as he gets involved in their adventures, he becomes more social and confident. He might also develop his character by exploring his own unique abilities and how they can be used for good.
A good way to create a compelling character for the twin brother is to make his powers have a cost. For instance, every time he uses a powerful Devil May Cry - style attack, it drains his chakra like in Naruto. This creates internal conflict as he has to balance when and how to use his abilities. Also, give him relationships that are complex, like having a love - hate relationship with a character from the Naruto world who is suspicious of his Devil May Cry powers, and at the same time, having a mentor - like figure from the Devil May Cry world who is trying to help him control his Naruto powers.
In some fanfics, the twin brother might have a special bond with a female character. For instance, Sakura could be intrigued by the twin brother's different personality compared to Naruto. She might initially be confused about her feelings towards both brothers, which could add a romantic sub - plot to the story along with all the ninja action.
The complex relationship between Itachi and Sasuke. Itachi had to make difficult choices for the sake of the village and his brother. Fanfictions allow readers to explore different scenarios where things could have been different, like if Itachi could have found a way to be honest with Sasuke from the start.
Maybe the twin brother was separated from Naruto at birth and raised in a different village. When they finally meet, they initially clash due to their different upbringings and loyalties, but then gradually come to understand and support each other. This would create a lot of drama and character development.
One possible plot could be that the twin brother has a very different personality from Naruto. Maybe he is more reserved and studious while Naruto is brash and energetic. The twin could secretly be training in a different type of jutsu, perhaps more focused on genjutsu while Naruto is into rasengan and other offensive ninjutsu. Their relationship might start off rocky as they compete for recognition in the village, but eventually they come to realize they are stronger together and support each other in battles against powerful enemies.
In this type of fanfiction, Sasuke could develop a greater sense of humor. With Naruto as his twin, he would be exposed to Naruto's antics more regularly, and some of that might rub off on him. Naruto, meanwhile, might become more strategic. He could learn from Sasuke's more calculated approach. Their combined influence on each other could also make them more diplomatic. They would need to represent their twinship well, and this would require them to be more balanced in their interactions with others, whether it's within the village or outside in the ninja world.
One possible plot could be that the twin brother has a unique power. Maybe he has a special chakra ability that neither Naruto nor Sakura possess. He could be a mediator between Naruto's brashness and Sakura's more level - headed nature. For example, in battles, he uses his power to help them out in unexpected ways, like creating a shield of chakra that can deflect attacks while Naruto attacks head - on and Sakura heals any wounds.
Naruto would adapt to Itachi's life in several ways. Socially, he'd have to get used to Itachi's connections and rivalries. For instance, his relationship with Kisame would be different as Naruto. He might try to influence Kisame in a more positive way. Emotionally, he'd have to deal with Itachi's internal struggles. Itachi had a lot of guilt and secrets, and Naruto would have to either come to terms with them or find a way to resolve them. And in terms of combat, he'd practice Itachi's signature moves over and over to make sure he could use them effectively in battle.