Mariel is one of the protagonists of No Place Feels Like Home. She is a clone of Nikki, from a timeline where she accidentally regenerated as a Time Lady after an accident with the TARDIS.
Mariel looks exactly the same as Nikki, apart from having naturally black hair and a different eye colour. She has a "gothic personality", but is for the most part cheerful. Her boyfriend is Dimitri, who she has dated since she was Nikki. The current Nikki doesn't know him.