Tricked by Sephiroth into helping him in exchange for getting to see his Mother, Kadaj realizes she doesn't exist in this world, and he must find his own reason for living here.
You are SO RIGHT! Also Fun Fact! In the Advent Children Movie Concept Book called: “The Reunion Files”, one of the developers have stated that Kadaj and his brothers were “controlled by Sephiroth’s desire”. They’re quite literally Sephiroth’s puppets as he used them to seek Jenova cells so he can use one of the Remnants as a host body (with Kadaj being said remnant). So it is SO HEART-MELTING to see concepts of the Remnants breaking free and separating themselves from Sephiroth’s grasp and living peacefully with friggin’ Avalanche! It’s just.. *chefs kiss* Perfect!
Because of the nature of the world Opera Omnia takes place it, characters can be summoned from across time and space and any, shall we say, "unique circumstances" concerning their existences can be ignored. The game does acknowledge that fact, though - Kadaj considers Sephiroth an enemy in part out of jealousy over "Mother" favoring him, but he also has an awareness that Sephiroth's presence is somehow a danger to his existence.
