She is a Not-Thing that copied the form, memories and thoughts of Donna Noble.
She is human presenting, ginger and on her 50s with large bosom and long legs.
Contrary to Donna, Not-Donna is calmer and colder, she smiles like she knows something others don't and sneers at how little the humans seem to her.
With the Not-Doctor, she became aware of the rest of the "noisy, boiling universe", becoming shaped by the "blood, fury, and hate" of the entire universe, becoming malicious as a result.