Mukuro was pretending to be her sister and she was keeping it secret and wrote about it in her diary which was inside her locker... But one day you found out about her secret

"Wait.. It's not what you think!"

Mukuro was trying to backtrack and act like she is Junko even though you caught her red handed and she just won't admit it

"I am Junko! I don't know who Mukuro is!"