Sōsuke Aizen
When Rukia is sentenced to death for her transgressions in the human world and taken back to the spirit world of Soul Society, Ichigo seeks out the assistance of Kisuke Urahara and Yoruichi Shihōin, who unbeknownst to him are two exiled Soul Reapers, to allow himself and his friends to save Rukia. After Ichigo and his friends battle many Soul Reapers, it is revealed that high-ranking Soul Reaper Sōsuke Aizen framed Rukia for the crime and has been illegally experimenting on Soul Reapers and Hollows. Aizen plans on taking over Soul Society via the use of the Hōgyoku, a legendary substance that can turn Hollows into half Soul Reapers and vice versa, increasing their powers greatly. He escapes into Hueco Mundo, the realm of the Hollows, and later abducts Orihime as she is instrumental in creating the Oken, a power that will allow him to kill the Soul King, the ruler of Soul Society.