I thought more about it when I saw a post someone else made about Emily having DID. I am not posting to attack that person, but simply to state my own opinion.
Most theories either say schizophrenia or bipolar. Those are both plausible, but Emily shows a combination of both disorders where symptoms co-occur, which brings us to schizoaffective disorder, bipolar type specifically.
Emily has manic episodes, where she makes reckless choices such as choking Kelly, sleeping with her friends' boyfriends or tattooing "Proud Mistake" on her back.
She also has depressive episodes, during one of which she makes a suicide pact with Nicole. She also has self-harm scars all over her body.
The schizophrenia symtoms show in the form of voices, hallucinations, unpredictable behavior and agression over perceived threats.
A differential diagnosis could be BPD, considering her history of unstable relationships, self-destructive behavior (criminality, promiscuosity, self-harm, drugs), emotional volatility, anger issues and impulsivity.
Emily could have both, but that's just my 2 cents in this discussion.