Since I haven't read the 'yasei no last boss ga arawareta novel', it's hard to say for sure. However, in a novel with this kind of title, we can expect a main protagonist who is either a regular person thrust into extraordinary circumstances due to the appearance of the last boss, or perhaps a pre - existing hero figure. There could also be a mentor - like character who provides guidance and knowledge about the last boss. And of course, there may be antagonists other than the last boss that the main character has to deal with along the way.
Without having read the 'yasei no last boss ga arawareta novel', it's difficult to name the exact main characters. But typically, in a story where a last boss appears, there will be a central character who has the courage and ability to take on this ultimate adversary. This character might be a young adventurer seeking to prove themselves, or an experienced warrior. There could also be a group of characters who accompany the main character. For example, a wise old man who has knowledge about the last boss, a strong and loyal friend who is always by the main character's side in battles, and maybe a mysterious figure who has some hidden connection to the appearance of the last boss.
I don't know as I haven't read it. But usually in such a story, there could be a brave hero or heroine who is likely to face the last boss. There might also be some sidekicks or allies that help the main character throughout the story.