Often, Naruto himself is one of the main characters facing these monsters. His determination and powerful jutsu make him a natural choice to take on such threats.
There could be a flying monster with razor - sharp claws. This kind of monster can swoop down on villages unexpectedly, making it a fearsome adversary for the ninja. It would force the ninja to develop new strategies to deal with aerial threats.
Sasuke could be one of his main allies. Despite their past differences, they could put aside their issues to support Naruto in his role as the King of Monsters. Sasuke's powerful combat skills would be a great asset in battles against those who oppose Naruto.
The main antagonists could be some of the traditional ninja clans who see Naruto's connection to the monsters as a threat to their power structure. They might fear that the monsters will disrupt the balance of power in the ninja world.
In addition to the regular Naruto characters, some of the more powerful monsters could be main characters too. For instance, a giant dragon - like monster that has the power of flight and can breathe ice. It could be a reluctant ally at first but then grow to respect Naruto and his friends. And among the aliens, there could be a leader - type character who is extremely intelligent and has unique powers that pose a great threat to the Naruto world.
A giant, hulking beast - type duel monster could also be very powerful. It might have incredible strength and endurance, able to take on multiple enemies at once. Its sheer physical presence could intimidate opponents, and it could use its brute force to smash through defenses and deal massive damage.
Sasuke is a top choice. His rivalry with Naruto can be intensified in the Naruto God of Monsters fanfiction. He might see Naruto's new power as a challenge to overcome. Then there's Gaara. His own experience with being an outcast and having unique powers can make him a great support character. He and Naruto could bond over their 'different' natures. And Jiraiya also pops up a lot. He could be the one to guide Naruto in understanding the ancient and mysterious aspects of his new power.
It could be a powerful ninja who has the ability to control the monsters. He might be using the monsters to achieve his own goals, like taking over the Hidden Leaf Village.
In a fanfiction, the aliens could capture Naruto and try to study his unique abilities. The monsters then decide to rescue him because they realize his powers could be the key to defeating the aliens once and for all. The story could involve a lot of action as the monsters infiltrate the alien base to save Naruto.
Perhaps some of the jealous monster students who don't want Naruto to succeed in school could be antagonists.
In Naruto Monsters vs Aliens fanfiction, it's hard to say who would win. Naruto's monsters have unique ninja abilities like chakra control and powerful jutsus. However, aliens might have advanced technology and strange powers. If the aliens' technology can disrupt chakra, they could have an advantage. But Naruto monsters are used to fighting in their own world's crazy battles.