The main themes include the human condition in a dark and brutal world. It shows how people can be both heroic and monstrous. Friendship and betrayal are also important themes. Characters form alliances that are sometimes broken due to various reasons, which adds to the complexity of the story.
Guts is definitely one of the most memorable. His tough and determined nature, along with his tragic backstory, makes him stand out. Another is Griffith. His charisma and the complex path he takes in the story are quite captivating.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.
Perhaps in this fanfic, Griffith has turned into something truly evil and a threat to the world Guts cares about. Guts, being the warrior he is, might feel it's his duty to put an end to Griffith to save others. Also, Guts could be driven by his own sense of justice. If Griffith has wronged innocent people, Guts may decide that killing him is the only way to set things right.