Looking like the Devil
Hiruma's general look can be summarized with just one word: flashy.
It's the kind of person you're bound to notice even if he's still at the end of the road: spiky blond hair, pointed ears, two pair of piercings, fangish grin and bubblegum always in his mouth make it impossible for you to ignore his precence.
His whole look is far from reassuring and makes him resemble a human-disguised devil. To further underline his demonic look, Murata tends to draw him with a pointed devil-tail and small horns when he's planning (or has already done) something particularly devious. (not to mention the "black evil aura of evilness" that seems to form around him whenever his evil genius gets in motion)
Everything of his general appearance is pointy and sharp: the shape of his face, his long pointed ears, his teeth - more similar to animal fangs than human teeth -, his spiked hair and sharp-shaped eyes and eyebrows. He's tall (from a Japanese point of view) and slim-builded, and his hands are characterized by thin and long fingers, with similarly long nails.
In the Devil's Clothes
Hiruma's usually seen either in his school uniform or in his football uniform, but, as sometimes the manga is setted outside the school or the field, he's also seen in normal clothes quite often.
Attending the Deimon High School, his school uniform is composed by black pants, white shirt and a turquoise jacket (according to the anime coloring).
During the series he shows two version of the football uniform (I can only guess they're the primary and secondary ones, considering that one was only used against the Bando Spiders which has a red-colored uniform similar to Deimon's other one). The first one is red in top and white in pants, the second one is red and white in top (being red the shoulders and chest sides, and white the front and back) and white pants.
When not in his school or football uniform, Hiruma's most frequent clothing is a pair of black tight pants and a equally black-and-tight T-shirt. He seems to prefer dark colors over lighter ones, though he once wore a quite-scary-and-hilarious-at-the-same-time pure white furred-collar jacket (with matching white pants) and an equally-quite-hilarious maid outfit to spy on USA team during the World League.