Donnie Darko is perceived by viewers of the film as being a classic anti-hero. He displays no consistent attributes that either a hero or superhero would normally possess but rather has glimpses of heroic feats that come about due to events out of his control. .
Donnie proves his anti-heroism on two separate occasions, the first which was flooding the school enabling him to meet the girl of his dreams was done physically by him but mentally he was unaware of his actions at all, the word anti-hero means a hero of unconventional type and isn't a hero that isn't awake during his actions incredibly unconventional. Donnie's actions enabled him to meet Gretchen and show her how "beautiful the world can be" but the style in which he did this surely proves him as an anti-hero.
The second of Donnie's Anti-heroic actions is the greater of the two. He subconsciously revealed a perverted child pornographer that was tricking the community into believing and spending money on him and his false products.
Once again all of this was done with out his knowledge and without him getting any recognition, but still it was Donnie who did it and the strange circumstances of the deed further illustrate that he is an anti-hero rather then a conventional hero.
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Some people might argue that Donnie's ability to see "worm holes" and potentially see into the future is a trait of a conventional super hero but this ability comes around only as random effect of the time tangent that the characters experience during the course of the film. His inability to control these wormholes make it an unconventional ability, which shouldn't classify him as a superhero but rather as an anti-hero. .
"A tangent is a sudden diversion from a course or direction.".
The perception we get of Donnie Darko as a character and human being is that he is mentally unstable, has an attitude problem and doesn't fit in with the school communities.