In Donnie Darko, during a failed attempt of a kiss, one fat guy is shown. The same fat guy is shown again in some random scenes. Is he some character from the film which I missed or is it some cameo? Or does this character signify anything?
The movie is pretty complex, so this answer mightn't make sense without a rewatch, but effectively the fat man is an FAA employee (Federal Aviation Administration). We know this as we see him among the group at both the start and end of the film.
The usual explanation is that the FAA are so perturbed by what's happened with the jet engine that they've sent an agent to effectively keep an eye on Donnie and the rest of the family.
From beginning of film:
From jogger scene: