The BLEACH universe is expansive and complex, with various configurations of the main states of being introduced over the course of the series becoming possible in part due to Ssuke Aizen's (and to some extent, Kisuke Urahara's) meddling with the boundaries between Shinigami and Hollow. Using the Hgyoku he extracted from Rukia's soul in the conclusion of the Soul Society arc, Aizen would go on to create more Arrancar, Hollows that have gained Shinigami powers from tearing off parts of their masks.