Amber does not technically have an alter ego. She just has some odd personality changes... 

One is Broken China-Doll, also known as China, where she'll simply become numb and has no morals.

The other is herself, Amber, the girl she was before she was taken by the Slender Man. In this state, she'll be a little bit more shy and polite than "China". 

In both of these states, she is still forced to kill others.