Bleach follows the adventures of Ichigo Kurosaki after he accidentally obtains the power of a Soul Reaper— a Japanese death personification similar to the Grim Reaper—from Rukia Kuchiki. Gaining these abilities forces him to take on the duties of defending humans from evil spirits and guiding departed souls to the afterlife.
Ichigo partially becomes a hollow in the same training. This leaves him with an insane inner hollow spirit that grows in strength as the series progresses and tries to take control of his body to fight. This control has manifests itself in several ways during the series, first as a berserker rage and later as a physical transformation. The inner hollow takes form as Ichigo's evil alter ego and mirrors his appearance.