It's possible that there is a villain - like character as well. In a fanfic about love and immortality, this character might be jealous of the main couple's eternal bond and try to break them apart. Their actions could drive the plot forward and test the strength of the main characters' love.
There could also be a supporting character who either helps or hinders the main couple's relationship. Maybe it's an old friend who has their own views on the immortality and love situation. This character could add some conflict or assistance to the story.
I'm not sure specifically as I haven't read the fanfic. But usually in a story about love, there might be two main protagonists who are in love with each other. They could be from different backgrounds, like one might be a supernatural being and the other a normal human, which would create interesting dynamics.
Typically, Ruby, Weiss, Blake, and Yang are still main characters in 'immortal rwby fanfic'. Their characteristics might be enhanced or changed due to the immortality factor. For instance, Ruby's leadership skills could be further developed over time. Weiss may use her long life to master more complex magic. Blake could have more time to fight for equality in different historical periods, and Yang's strength and determination might reach new levels as she has all the time in the world to train.
If it follows the typical pattern of a story, aside from the immortal dragon, there might be a human or elf character who is either an ally or an enemy of the dragon. There could also be other magical creatures that play important roles in the fanfic, like a wise old wizard or a powerful demon that the dragon has to face or work with.
Batman is obviously one of the main characters. His presence is central as the story is likely based around his immortality. Robin could also be a main character, as he is often Batman's sidekick and their relationship would be different with Batman being immortal.
It could also be that an original character created for this fanfic is the main focus. This character, being immortal, would have their own backstory and motives that are central to the plot. They might befriend or oppose Naruto and his friends, leading to various adventures and conflicts.
Typically, in a fanfic that combines Naruto and Bleach elements and involves immortality, you would expect to see Naruto Uzumaki. He is a central character in Naruto and his story could take a whole new turn with immortality. Also, Ichigo Kurosaki from Bleach would probably be there. His unique powers and his journey could be greatly affected by the concept of immortality. There might also be other important characters from both series, like Sasuke from Naruto or Rukia from Bleach, who play important roles in the new narrative of the fanfic.
I don't know the main characters in this fanfic as I haven't read it. But in MLP, common main characters could be Twilight Sparkle, Pinkie Pie, Rainbow Dash, etc. Maybe this fanfic has a new or less - known character as the main focus along with some of the regular ones.
Often, there are two central characters. One might be an ordinary human with a pure heart, and the other could be an immortal like a vampire or a fairy. For example, a young girl who stumbles upon a hidden world of immortals and falls in love with a centuries - old vampire.
Since I don't know the details of this 'Chord Immortal Love Story', I can't say for sure who the main characters are. It could be a musician and his or her beloved.
I'm not sure as the title doesn't give that away. It could be two best friends, a male and a female perhaps.
There's also a possibility that the main characters are a mix of well - known characters from the source material and new ones. For example, if it's a fanfic based on a TV show, it could be the show's main couple along with a new character who is like a guide to help them understand the different 'colors of love' that they are starting to experience.