Diana Napier, born 05-21-1995 in Liberty City, is the niece of Jack Napier.

At first, Diana had a quite normal childhood, until her parents, Bernard and Zoe Napier, lawyers in Liberty City, died in an accident when she was 14 years old. As her uncle, Jack Napier, had broken the contact to her parents about one year earlier without any obvious reason (although it can be assumed he had a problem with them being lawyers and himself being a crook), she was now on her own. The next years find her moving from foster parents to foster parents, until she got into trouble with the law and ended up in Alderney State Correctional Facility after a shootout in the Triangle Club (no, she wasn't working as a stripper there - she dealt with drugs, from the view of her uncle the better choice).
Jack Napier paid his attorneys, Goldberg, Ligner & Shyster, who have their headquarters in Liberty City too and only a dependance in the Arcadius Business Center in Los Santos, to get her out of prison and move to Los Santos.
Of course, she immediately asks him, why he didn't care for her for eight years (which means, that she isn't, as he guesses, 25 at the time of the events of Finance & Felony - The Movie, but 21), but his answer is evasive, he states that he “had a lot of troubles” and didn't want to involve her. But of course, he immediately involves her into his businesses, when he sends her checking on his old meth lab and generally start working for him.
Nowadays she's overseeing Jacks business, now officially a firm called Napier Incorporated, in Liberty City.

Finance & Felony - The Movie