It could be Naruto himself. Imagine Naruto with the power to summon kaiju. He would be even more of a force to be reckoned with. His usual knack for making friends could extend to the kaiju he summons, and he could use their power in his never - ending quest to protect the Hidden Leaf Village and bring peace to the ninja world.
A new character could be the main character. Maybe an outcast ninja from a small, unknown village. This ninja has always been looked down upon because they don't have any strong bloodline abilities. But then they discover the power to summon kaiju, which completely changes their status. They start off using the kaiju to prove themselves to their village, but then they realize there are greater threats in the ninja world that they can use their newfound power to fight against.
Well, it could be about a ninja who discovers the power to summon kaiju in the Naruto universe. Maybe they use these huge creatures to fight against powerful enemies in battles that are much larger - scale than the typical ninja skirmishes. For example, they might summon a kaiju to take on a powerful rogue ninja who has an army of summoned beasts.
The main character could be a young orphan who discovers they have the power of a summoner. They grow up in the Naruto world but have always felt out of place. One day, they accidentally summon a pony and realize they have a connection to another world. This character would be on a journey to discover their true identity and the full extent of their summoning powers.
It could be a new genin who has always felt different from the other ninja in their class. This character discovers their kitsune summoning ability during a training exercise and then embarks on a journey to master it.
Another option could be a character who was once a villain or a criminal but is given a second chance through the power of chimera summoning. They use this new ability to redeem themselves and fight for a different cause, which would make for a very interesting character arc.
It could be a young genin who has always been interested in spiders and discovers their arachnid summoning ability. Maybe they were orphaned and had to rely on their own wits and new - found power to survive in the ninja world.
Arya Stark could be an interesting main character as a summoner. She already has a lot of skills like stealth and combat. If she were a summoner, she could summon the spirits of those she has killed to either help her in battles or to gather intelligence. It would be a new twist on her character.
It could be another summoner who is jealous of the panther summoner's power. This antagonist summons a powerful and dangerous beast, like a giant snake, to constantly challenge the panther summoner and try to steal their power or reputation.
The main antagonist could be one of the village elders. They might see Naruto's fox - summoning abilities as too dangerous and banish him out of fear. Throughout the fanfiction, they could try to prevent Naruto from coming back to the village, fearing that he'll seek revenge or disrupt the power balance they've established.
Maybe the character initially struggles with the fact that summoning a hydra is seen as an evil or dark art in the Naruto world. As they progress, they try to change the perception of others about hydra summoning by using it for good, like protecting the village from greater threats. This way, they develop not only in their skills but also in their standing within the community.
Obviously, Naruto would be one of the main characters. Since it's a fanfiction about him as a dragon summoner. There might also be his friends like Sasuke and Sakura. Sasuke could be intrigued by Naruto's new power and might have a complex relationship with it, either being competitive or even learning from Naruto in some way. Sakura could be worried about the risks Naruto takes with this new and dangerous power.