For Naruto, you could explore his growth as a leader further. In a fanfiction with Amaru, he could teach her about the importance of loyalty to the village. Amaru, on the other hand, could develop by learning from Naruto's never - give - up attitude. Maybe she faces a personal crisis where she wants to give up, but Naruto's example inspires her to keep going.
To develop their characters further, put them in a situation where they have to rely on each other's weaknesses. Naruto's over - eagerness could be balanced by Amaru's more cautious nature. Amaru might be too hesitant at times, and Naruto could encourage her to be bolder. Through these interactions, they both grow as characters. For example, in a battle situation, Naruto's initial rush might get him in trouble, but Amaru's calculated moves could save him, and vice versa.