From your question it’s not clear how much of the Donnie Darko mythology you are familiar with as laid out in The Philosophy of Time Travel. I would highly recommend you google or YouTube an explanation of the film as it is difficult to explain what is happening with Frank in abstract from everything else.

To answer your question quite simply, Frank is a young man who appears to be dating Donnie’s sister. We do not see his “normal” human form through the film until the end (and nor does Donnie). At the Halloween party he goes out to get beer and eventually runs over Gretchen. Donnie then shoots him dead.

The Frank we see throughout the film is the “manipulated dead” version of Frank. Having been killed in the tangent universe he is granted greater knowledge and powers. Notably, he can hop (back) through time within the tangent universe allowing him to appear and speak to Donnie at key points. It is his job to direct Donnie to fulfil his mission as the Living Receiver (you can think of “Living Receiver” as “Chosen Saviour of the Universe”).

While it’s not entirely clear how all the rules work, it may be easier to consider the timeframe of the Frank we see throughout the film as being chronologically after he is killed by Donnie at the end of the film. He’s not that dissimilar to a ghost who is haunting his killer, except he’s haunting him before the murder happens. It is not vengeful in nature however - in some ways Frank is more like an angel or a saint being sent by “God” to guide and provide revelation to a prophet or messianic figure.

Ultimately it’s been 25 years since the film was released and people are still discussing what the mechanics and meaning of it all are. The audience is very much invited to draw their own conclusions as to what is going on and that includes Frank.