Definitely Hinata. Since Hinata idolizes the Little Giant, a fanfiction with them together could be amazing. They could have training sessions where the Little Giant passes on his skills to Hinata. Another good character to pair with the Little Giant could be Kageyama. Their different playing styles could create an interesting dynamic in the story. And of course, Daichi. As the captain, he could learn a lot from the Little Giant's leadership on the court.
One common theme is inspiration. The Little Giant serves as a huge source of inspiration for the Karasuno players, and this is often explored in the fanfictions.
There are various types of interesting Haikyuu Little Giant fanfiction. For example, a story that details his first match and how he overcame his nervousness. It could describe his thoughts during the game and how he managed to turn the situation around. Another could be a fanfiction where the Little Giant has an unexpected encounter with an opposing team's star player outside of the court and they have a deep conversation about volleyball. Also, a story where the Little Giant mentors a young player who is having self - doubt could be really engaging.
There is a fanfiction called 'In Search of the Little Giant'. It follows a new player who is inspired by the tales of the Little Giant and tries to find out more about him while also improving his own volleyball skills at Karasuno. This fanfiction has some really interesting character development and a great plot that keeps you hooked.
Since I don't know the exact 'giant little brother story', it's hard to say. But most likely, the main character is the little brother who has the 'giant' aspect. There might also be family members like his parents or siblings involved.
Since I don't know the exact story, I can only assume that the main character is the little girl. But there could also be other characters like her family, friends, or people she meets during her adventures as a 'giant' or in a world related to giants.
As we don't have much information about this 'little giant girl story', the main character could be a fictional girl created to teach a lesson, like the importance of believing in oneself. She might be a girl from a specific cultural background that adds to the uniqueness of the story.