Kirishima's story

What was Eijiro Kirishima story?

Before Horikoshi reveals it completely, I decided I'd try to piece it together. It's speculation mixed with stuff we already know.
Black-haired Kirishima began with a much less durable hardening Quirk, was bullied for one reason or another, and resented himself for his weakness. But one day, he came across videos of an old school hero named Crimson Riot. He heard something about Riot's story that inspired him to become stronger, maybe Riot said something inspirational about hard work and manliness.
So he trained day-in and day-out, getting to the point where he was able to pass the U.A. entrance exam. Before entering U.A. proper, he decided he would change his image: gone was the weakling who everybody picked on. He decided he would become the cheerful redhead that we know today, styled after his idol Crimson Riot.
Kirishima thought that hanging out with strong people would make him even tougher, so that's what gravitated him towards Bakugo, the straightforward tough guy. Thus, a manly friendship was born.

how does Kirisima look like?

Eijiro is a muscular young man of average height and a rather impressive physique for his young age. He has red eyes that are pointed slightly inwards and a small scar just above his right eye, which Horikoshi admits he frequently forgets to draw. He also has small eyebrows and very pointed teeth. His hair is reasonably short, dyed a bright red, and spiked away from his head at all angles with gel, two more pronounced tufts spiked on either side of his forehead like little horns. This lofty style takes a full three minutes to set.

During middle school, however, his hair was black and left in its natural state: just under shoulder-length with some shorter bangs between his eyes. He changed it to his current style shortly after being accepted into U.A. High School.

During school, he wears the normal U.A. uniform, just with dark red trainers instead of the usual brown dress shoes.

In his hero costume, he wears two dark red gear-shaped shoulder pads. A jagged sash joins the one on his left to the right side of his belt, which has a red "R" set into its center, standing for his Hero name, "Red Riot". Below this, he wears baggy black pants and half cape with a ripped hem. He also sports dark red boots and multiple thick rings going around his calves. He leaves his chest bare, and wears a wired guard around his face, reaching from just above his hairline to below his jaw, an extra piece going over the bridge of his nose, and some spiked pieces around his mouth which resemble little fangs. This look overall slightly resembles an Oni, which is a troll-like creature commonly seen in Japanese folklore, but whether this was purposeful or not hasn't been confirmed.

During the Provisional Hero License Exam Arc, his costume later gained black sleeves, which he wears to prevent civilians from being hurt by his Quirk during rescues.