One way could be by creating a character with a unique immune system ability. For example, a character who can heal rapidly due to an enhanced immune system, which could be a result of some experiment or natural mutation in the RWBY universe. This could add an interesting dynamic to battles and storylines.
You could have a character's immune system be linked to their Aura. Since Aura is a form of energy that protects the characters in RWBY, it could be that a strong Aura also means a strong immune system. So when a character's Aura is depleted, they become more vulnerable to diseases or toxins.
In RWBY fanfiction, the immune response could be depicted in various ways. It might be related to how the characters deal with a new type of Grimm toxin. For example, some fanfics could show a character having an enhanced immune response due to their unique Aura training, which helps them fight off the negative effects more quickly than others.
Often, it's portrayed through the characters' ability to resist certain negative effects. For instance, if there's a disease - like condition caused by some dark magic in the fanfic, characters with stronger Auras might show a better immune response by not getting as sick or recovering faster.
It could be related to some kind of magic protection. In the wizarding world, there are various protective spells and enchantments. Maybe Harry has acquired or was born with a special protection that makes him immune to allure in this fanfiction.
There could be various such stories. Maybe they face challenges in daily life due to their vulnerable immune systems and have to take special precautions.
It also means he is more reliable for his friends. Since he is immune to allure, his friends can trust that he won't be swayed into making bad decisions that could endanger them or their cause. He can be a stable and trustworthy figure in the fanfiction's world. For example, when facing a difficult situation where there are temptations to betray the group, he will firmly stand with his friends.
One implication is that he becomes a more formidable opponent in battles. Enemies who rely on poison - tipped weapons or poison - based jutsus would be at a disadvantage against him.
Yes, in certain fanfictions Naruto is immune to fire. This can add an interesting twist to the story. For example, if he's facing a powerful fire - using enemy, his immunity gives him an advantage. It also allows for new plot developments and character interactions that wouldn't be possible in the original story.
Well, usually the main character is the one who gets reincarnated with the system. They are often an outsider to the RWBY world at first and have to figure things out. Then there are likely to be the main RWBY characters like Ruby, Weiss, Blake, and Yang who will interact with the reincarnated character.