It's called Dont fall asleep in it tai (and later his sister) would fall asleep and wake as a different version of himself in a different reality. Such versions include being a younger brother of kari, baby, female self. He would become biyomon in the last few chapters.
http://www.fanfiction.net/s/685662/1/Dont_fall_asleep
Check out the author has two other fanfics of interest.
I'd like to see art depicting scenes in it but you probally need the author's permission to do that.