During the film Donnie Darko, Donnie occasionally talks with Frank, who we end up finding out at the end of the film is a guy that Donnie shot because he accidentally ran over his girlfriend. Is Frank a manifestation of Donnie's consience, having premonitioned what was going to happen in the future? If not, what was Frank supposed to be?
The real-life Frank that runs over Donnie's girlfriend is Elizabeth's (Donnie's sister) boyfriend. We see him drive past Donnie at the start of the movie after dropping her off at home shortly before the engine crashes into the house. You can see this sequence of events more clearly at the end of the film.
The Frank that Donnie speaks to throughout the movie is a Messenger sent to warn Donnie of the impending end of the world. The characters that are killed in the alternate universe are manipulated by whomever is helping Donnie save the world. They are known as Manipulated Dead and are briefly explained in the book from the director's cut.
Who is Frank?
CHAPTER : THE MANIPULATED DEAD
The Manipulated Dead are more powerful than the Living Receiver. If a person dies within the Tangent Universe, they are able to contact the Living Receiver through the Fourth Dimensional Construct. The Fourth Dimensional Construct is made of Water. The Manipulated Dead will manipulate the Living Receiver using the Fourth Dimensional Construct. The Manipulated Dead will often set an Ensurance Trap for the Living Receiver to ensure that the Artifact is returned safely to the Primary Universe. If the Ensurance Trap is successful, the Living Receiver is left with no choice but to use his Fourth Dimensional Power to send the Artifact back in time into the Primary Universe before the Black Hole collapses upon itself.
I mentioned that Frank was from the future. Frank is also dead. Doesn’t makes sense? Ok, Frank is a Manipulated Dead. Manipulated Dead is a person who has died in the Tangent Universe. This person, after death, has the ability to travel back in time and guide the Living Receiver to get the Artifact back to the Original Universe - the Primary Universe. At some point in the future (in the Tangent Universe), a character called Frank dies. We’ll get to who Frank is and how he dies later on. Frank is now a Manipulated Dead and travels back in time to meet Donnie. Frank begins guiding Donnie towards his mission. Donnie’s mission is transporting the Jet Engine to the Primary Universe before the Tangent Universe collapses. This mission will ensure the Primary Universe doesn’t collapse too. Dead Frank, along with the Living, constantly guide Donnie towards various actions. Each of these actions form a trail culminating at the goal. Does Donnie know what his mission is? No he doesn’t, he learns it through the course of the events. He is mostly playing along.
Frank (who before dying is a regular kid) is the Manipulated Dead who exists only in the Tangent Universe. (Dead) Frank comes back in time and ensures that Donnie goes on to do his bit to "fix" the glitch of the 4th dimension.