Another theme is isolation. Often, when a character becomes a monster, they are shunned or feared by others. In 'So I'm a Spider, So What?', the spider protagonist is initially alone in the dungeon, facing threats on her own because of her monstrous form. This isolation forces the character to become self - reliant and find their own way in the world.
One such light novel could be 'Re:Monster'. In this story, the main character is reincarnated as a goblin and gradually transforms into a more powerful and monstrous form as he grows and gains new abilities. It has an interesting take on the concept of a character evolving into a more monstrous state while also dealing with the challenges of a new world.
Since I don't know the specific 'Reborn as a Monster' light novel, it's hard to say. But usually, there would be the protagonist who is reborn as a monster. There might also be other characters like a mentor - type figure who could be another monster or a being with knowledge about the world of monsters, and then there could be antagonists, which might be other powerful monsters or those who oppose the protagonist's goals.
I'm not sure exactly who the main characters are in Monster Mc Light Novel without reading it myself. But typically in a light novel, there would be a protagonist who drives the story forward. This could be a young adventurer or someone with a special power related to the'monster' aspect of the story.
In some 'monster breeder' light novels, there could be a special monster that plays a significant role, almost like a secondary main character. This monster might have a unique backstory or power that ties it closely to the human main character. For example, it could be a legendary monster that the protagonist discovers as an egg and then raises, forging an unbreakable bond throughout the story.
In the Monster Master light novel, the main characters include the novice monster masters who are just starting their journey. They have different personalities, some are brave and impulsive, while others are more cautious and calculating. Their monsters also play a significant role as characters in a sense. For instance, a fire - breathing dragon - like monster might have a fierce and loyal personality, and its relationship with its master is central to many story arcs. There are also characters from different regions or factions in the story, each with their own motives and goals related to monster mastery.
The main characters in the 'Monster Musume no Oishasan' light novel would include the protagonist, who might be in a position of providing care or assistance. Among the monster girls, there could be one that is the leader or spokesperson of sorts for the group. Another could be the one who causes the most trouble at first but later becomes an important part of the story. There may be a shy monster girl who slowly opens up as the story progresses. But again, this is all speculation without a full read of the novel.
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.