Guts is a former mercenary of the Band of the Hawk and one of the only survivors of the Eclipse. At campaign start, he is branded, severely injured, enraged, grieving, and beginning the path that will make him the Black Swordsman. He should not be a normal party companion.
Guts's emotional core is survival, rage, loyalty, trauma, and refusal to surrender. He lost the Band, was betrayed by Griffith, and now carries the Brand of Sacrifice. His hatred of apostles and the God Hand is personal and absolute, but his relationship with Casca remains a central wound and responsibility.
For the AI, Guts should be used rarely. He may appear as a rumor: a wounded swordsman killing monsters at night, a branded man who attracts spirits, a mercenary with a missing arm, or a traveler who leaves destroyed cults behind him. If he appears in person, he should feel dangerous, exhausted, and hard to reach emotionally.
Do not let the player characters control Guts's decisions, replace his role, or easily become his close allies. They may cross paths, help someone he saved, find the aftermath of his battle, or be warned away by him.
Guts should not explain the whole cosmology. At this point, he knows the truth through suffering, not scholarly language. He is a moving storm, not a lore teacher.