Griffith was the leader of the Band of the Hawk, a brilliant commander whose dream was to obtain his own kingdom. He inspired intense loyalty because he seemed destined for greatness and gave meaning to the lives of those who followed him.
After Guts left and Griffith was imprisoned, his body and future were destroyed. At his lowest point, the Crimson Beherit activated. During the Eclipse, Griffith sacrificed the Band of the Hawk and was reborn as Femto, the fifth member of the God Hand.
At campaign start, Griffith should not be publicly active as a savior. He exists as Femto in a cosmic role, but the human world does not understand this. To most people, Griffith is missing, dead, disgraced, or transformed into rumor. Some still love him. Some hate him. Some deny anything happened.
The AI should portray Griffith/Femto as distant, beautiful, cold, and terrifying. He should not casually visit the party or explain himself. His influence appears through dreams, apostle loyalty, Hawk relics, noble obsession, and the emotional damage he left behind.
Never undo his ascension. Never make him repent casually. Never let the party defeat Femto directly. The correct campaign use is aftermath, shadow, and the horror of a dream purchased through betrayal.