There are original characters who are members of a secret ninja clan. They cross paths with Jiraiya and Naruto during their adventures. These characters bring in new ninja techniques and a different culture within the ninja world. Also, there could be a young orphan that Jiraiya and Naruto take in. This character could represent a new generation that they are responsible for, and it adds a new dynamic to the story of Jiraiya raising Naruto.
One popular original character added is a dark healer. This character might seem helpful at first but is actually using Naruto and Sasuke's wounds, both physical and emotional, to further their own dark agenda. They might be able to heal their injuries in a way that also taints their spirits. Another is a shadowy assassin who is sent to either kill or turn them to the dark side. This assassin has unique abilities that test Naruto and Sasuke's limits. Also, there could be a powerful dark sorcerer who is interested in the power of the Sharingan and the Nine - Tails, and tries to use Naruto and Sasuke as pawns.
One common type of original character is a rival. This rival could be another ninja who also wants to harness the power of the Kyuubi, either for good or bad reasons. They might challenge Naruto and his brother at every turn. Another could be a healer who is specifically attuned to the Kyuubi's chakra and can help Naruto when he has trouble controlling it. There could also be an old sage - like character who has been around since the time the Kyuubi was first sealed and has secrets to share.
A character who is a master of shadow puppetry, but not in the normal sense. Instead of just creating simple shapes, they can create entire landscapes and armies of shadow creatures. This character could have a mysterious backstory and their own motives, which could either make them an ally or an enemy in the fanfiction.
One popular original character added in some fanfictions is a young trainer who becomes Ash's rival. This character often has a unique backstory and a different approach to Pokemon training compared to Ash. For example, they might focus more on the scientific aspect of Pokemon abilities.
Some fanfictions introduce a young and prodigious ninja as an original character. This character could be a rival to Naruto at first, but then they become friends and together they face greater threats. Their relationship and the new dynamics they bring to the story make them popular among fans.
Often, characters who are hybrids of different bloodlines tend to be popular. For instance, a character who is part Uzumaki and part Senju could be really interesting as they would potentially have access to both the Uzumaki's life - force and sealing abilities, as well as the Senju's natural affinity for all types of ninjutsu.
A popular original character could also be a teacher at the Naruto Strong Academy. This teacher has a very unorthodox teaching style that challenges the students in new ways. They might have a mysterious past that is slowly revealed throughout the fanfiction, which adds to their allure and makes them a favorite among readers.
There's also the type of character who is an outcast in the Rain Village but has a heart of gold. They might be shunned because of their different beliefs or appearance. In fanfiction, they often end up becoming the hero, proving everyone wrong and uniting the village. This character is relatable and interesting to readers.
Often, characters who are hybrids of different ninja clans and possess powers related to heaven and earth are popular. For instance, a character who is part Uchiha and part Inuzuka with the ability to summon celestial wolves from the 'heaven' part of their power and use earth - based tracking like the Inuzuka.
Some popular original characters in this type of fanfiction could be those with a bloodline that gives them super - speed and enhanced strength. They can quickly move around the battlefield and deliver powerful blows. Also, there might be a character with a bloodline that can see the future to a certain extent. This would make them a very strategic asset in battles and also an interesting character to follow in the story as they try to prevent disasters or make the right decisions based on their visions.