#1 Beginning

Adel Yeager is Eren's older brother by six years. He and Carla is special because of what you call Guardian or Spirit animals.
Eden did not get the genes but Carla and Adel are the only ones left, they hid there secret from Eden and Carla took it to her grave.
Carla was the spirit animal of the golden eagle but since Adel is her offspring he is the spirit of the wolves and the golden eagles.
Adel goes missing after the walls are breached and that is where we start, with one of two chapters when Adel was younger.

PS. I do not own Attack on Titan's only Adel