He is voiced by Toshiki Masuda in the Japanese version of the anime, and by Justin Cook in the English version of the anime.
Eijiro is a muscular young boy of average height, with a rather impressive physique, despite his young age. He has red eyes that are pointed slightly inwards, 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 taking a full 3 minutes to set.
In his middle school days, however, his hair was black and left in its natural state: just under shoulder-length with some shorter bangs between his eyes, but 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 gear-shaped shoulder pads, dark red in color, a jagged sash joining the one in 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, also sporting black boots, 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 makes him look a little like 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 gains black sleeves, which he wears to prevent civilians from being hurt by his Quirk during rescues.
Eijiro is a boisterous and outgoing boy with a fondness for the concept of manliness, often using the terms "manly" and "unmanly" to describe things and people he does or doesn't like. Those who showcase a noble, brave, and/or determined attitude tend to emotionally move him to tears, quickly earning his complete admiration, respect, and friendship. Similarly, he will call out traits that are the opposite of that, such as cowardice and dirty play. He is also a big fan of Crimson Riot, a retro hero that he takes inspiration from, including his own hero name.
Eijiro is a very honest and kind-hearted boy, with All Might praising his heroic spirit. In addition, he seems capable of making friends with most people, as he is one of the few classmates who is friends with both Katsuki Bakugou and Izuku Midoriya. Eijiro is very dedicated to his friends, especially Katsuki (who he's very loyal to), and is even willing to break laws and rules in order to help them, showing regret if he becomes unable to aid in any way. This sometimes leads to Eijiro being very hard on himself.
Eijiro has stated more than once that he does not think very highly of his own Hardening Quirk, thinking that it lacks the flashiness usually found in the Pro Hero scene. In fact, he will get downright offended if he finds someone with a similar Quirk. Eijiro also has concerns about not having a good enough Quirk for hero work, worried that his skills could not compare to the others'. A lot of this self-doubt seems to have come from an incident in Junior High when Eijiro saw Mina Ashido stop an underclassman from being bullied and save some students from a threatening figure. Eijiro's friends pointed out that Mina will easily become a Hero due to her outgoing personality and athletic nature. After this, Eijiro thought of himself as dull and boring, considering himself pathetic for his inaction and powerlessness.
Regardless, in the present, Eijiro decided to reinvent himself after being accepted into U.A. High, adopting a tough, confident, fiery persona, although not to the same extent as Katsuki. Eijiro, however, does not shy away from comparing himself to him, with Izuku stating that the two have managed to develop a relationship between equals. Because of this, Eijiro is notably one of the few people that Katsuki holds in higher.