The next day, Rukia appears in Ichigo's classroom as a transfer student. Much to his surprise, she appears to be a normal human. She theorizes that it was the unusual strength of Ichigo's spirit that caused him to absorb almost all her powers, thus leaving her stranded in the human world. Rukia has transferred herself into a gigai — an artificial human body — while waiting to recover her abilities. In the meantime, Ichigo must take over her job as a Soul Reaper, battling hollows and guiding lost souls to Soul Society.
-Synopsis is from Wikipedia.