Another possible main character could be a native of the water world who befriends the reincarnated character. They could show the protagonist the ropes of this new world, introduce them to the different cultures and ways of life. Maybe it's a young water spirit who is curious about the reincarnated human and wants to learn about the outside world through them.
There might be a wise old mentor figure. In many light novels, there's someone who helps the protagonist understand the new world they're in. In this case, it could be an elder who has a deep understanding of water magic or the secrets of the water - based world. And there could also be an antagonist, perhaps someone who wants to disrupt the balance of the water world for their own gain.
Often in this type of light novel, the reincarnated character has friends or allies among the elves. These could be young elves who are curious about the new ideas the main character brings. And there could be an elder elf who has great wisdom and guides the main character through the intricacies of elven culture and magic. These are all potential main characters in the story.
The sword, which is the reincarnated entity, is a major character. It has its own personality and goals. The girl who teams up with the sword is also very important. She has her own backstory and abilities, and together they face the challenges in the story. There may also be some other side characters who play important roles in their adventures, like the various monsters or other adventurers they encounter.
The main character is the one who gets reincarnated as an alchemist. His name might vary depending on the specific version, but he's the central figure. There are also often his mentors or teachers in the world of alchemy. They play important roles in guiding him on his journey to master alchemy. And then there are his friends or comrades he meets along the way, who join him in his adventures and challenges.
There might be a human character who befriends the reincarnated demon. This human could be a mage or a warrior who sees past the demon exterior and forms an unlikely alliance. Another possible main character could be an angel who is on a mission to either convert or destroy the reincarnated demon, adding a lot of conflict to the story.
The most prominent character is usually the protagonist who has been reincarnated as an aristocrat. Their name varies depending on the specific story, but they are typically the central figure driving the plot forward through their actions and decisions.
The main character is Rimuru Tempest. He was a human in his previous life and got reincarnated as a slime. Another important character is Veldora, the Storm Dragon who becomes Rimuru's friend.
Rimuru Tempest is the main character. He is the reincarnated slime who drives the whole story forward with his adventures and growth.
Myne is definitely the central character. Her character is complex as she has the memories of her past life and her new life's desires and limitations. Her relationships with the other characters, such as the nobles she encounters in her pursuit of books, are also important aspects of the story. And of course, her family in the new world, which includes her father and sister, are important in shaping her character and the story's development.
The protagonist, who is the reincarnated villainess, is of course a main character. She's the one driving the story as she tries to change her fate. Then there's usually a male lead, who is often part of the original story that she's now trying to rewrite. There's also the original heroine, with whom the protagonist has a complex relationship as she's now in the role of the villainess against her. These three are typically the main characters that shape the story.
Typically, the main character is the one who has been reincarnated as a monster. Their personality might be a mix of their former self's traits and the instincts that come with their new monstrous form.