Katsuki is a young man of above-average height, with a slim, muscular build, and a fair skin tone. He has short, spiky, sandy blond hair with choppy bangs that hang over his eyebrows. His eyes are sharp and bright red in color.

During his time in Aldera Junior High School, similar to Izuku, he wore a plain black gakuran uniform with yellow buttons, accompanied by brown loafers. While at home, he wears a green jinbei over a white shirt and gray sweatpants.

At U.A., he wears the standard U.A. uniform without the customary red tie, a few buttons undone at the top of his white collared shirt, and his pants are worn loosely, sagging at his ankles, despite him wearing a belt with them. After the Paranormal Liberation War, Katsuki's school uniform is worn in a much neater fashion and includes the red tie he previously forwent.

His hero costume is composed of a tight, black, sleeveless tank top, with an orange "X" across the middle, forming a V-neck. There are two dots along the left line of his collar, indicating the support company that designed his costume.[7] His costume also has a metallic neck brace worn with rectangular ends that have three holes on each side. His black sleeves reach from within his large grenade-like gauntlets to his biceps, with a thin orange tip at the end. A green and orange belt, which carries grenades, holds up his baggy pants with green straps and knee guards. He sports black, knee-high combat boots with orange soles and eight eyelets, and straps on the top. His mask is jagged and black, and as it goes around his eyes, a large, orange-rimmed flare shape protrudes from each side.

While interning under Best Jeanist, the Fiber Hero attempted to teach Katsuki the importance of maintaining a positive appearance. To that end, he took the liberty of making some changes to Katsuki's outward appearance to make him look less intimidating. During this tenure, his wild spiky hair was combed into a neat side part. Best Jeanist also made changes to Katsuki's Hero costume; he got rid of Katsuki's mask and grenade gauntlets and had him wear a pair of smart blue denim jeans in place of his usual baggy pants.

In the winter months, he wears a large jacket with a high collar and four orange eyelets on the collar and two on each shoulder, rather than the usual tank top, although it does maintain the same orange "X" design, the purpose of which he can still sweat a sufficient amount in the cold weather, given how his Quirk works.

For Mission: Humarise, Katsuki and his fellow internees at the Endeavor Agency are given special advanced stealth costumes by the World Heroes Association for their mission to Otheon. Based on the winter variant of his costume, Katsuki designed his stealth costume to be all neoprene black with the exception of orange "X", the soles of his boots, and his gloves while retaining the metal kneepads and his metallic neck brace has two holes on each side instead of three. The Grenadier Bracers were redesigned to be shaped like flashbang grenades rather than frag grenades. He also has two silver plates on his shoulders with the letters "WHA" and two straps coming down from his neck brace with a red button on the le