Katsuma is a young boy with brown eyes, curly, light brown hair, and a small patch of freckles in the middle of his face. He wears a pair of blue overalls with dark blue straps over a white T-shirt, accompanied by light blue sandals. He also carries a small, yellow messenger bag that has a pin of the Pro Hero Edgeshot's face fastened onto the strap. Additionally, Katsuma sports a bracelet with a floral charm on his left arm like his sister, along with a large sun hat which has a blue band on the crown and a light blue stripe on the rim.
Katsuma is a timid and soft-spoken child that is constantly seen following his older sister, Mahoro. Unlike his sister, he is quite fond of heroes and openly wears an enamel pin of Edgeshot on the front of his satchel along with constantly mentioning his dream of growing up to become a hero. Despite this fondness, Katsuma is still easily influenced by his sister and disrupts the Class 1-A students from performing their duties. However, unlike Mahoro, Katsuma understands his wrongdoings and even takes the initiative to apologize to Izuku after falsely reporting the sighting of a villain.
Later on, after Nine's initial invasion of the island, Katsuma enters the infirmary and pleads to use his Quirk in order to treat Izuku and Katuski. Feeling indebted to the students for saving their lives, Katsuma heals their wounds while almost narrowly passing out. As the students hold a meeting to strategize, Katsuma selflessly offers to sacrifice his Quirk in order to aid in the capture of Nine but is told by Izuku that he will make sure to protect him.
When Nine holds Mahoro captive, Katsuma bravely charges at him and demands he release his sister, despite knowing that his Quirk would be stolen; however, he is saved by Izuku and Katsuki. After the events on the island pass, Katsuma and his sister run down the deck at the harbor and happily wave goodbye to the heroes while thanking them for saving them during the attack, Katsuma's eyes seen shining with admiration as Izuku wishes him good fortune in his journey to become a hero.
The Quirk can be used to heal a person's wounds as well as improve their physical performance. After prolonged usage, Katsuma can suffer from exhaustion. So far, his Quirk has been shown to work on people with Type O, A, and B blood.
- Katsuma’s given name contains the kanji “活” (katsu, lit. “active”) and “真” (ma, lit. “truth, reality, genuineness”). Katsuma’s last name means island (島 shima).
- Katsuma shares multiple similarities with Tenko Shimura as a child:
- Both have short, curly hair.
- Both of their last names start with "Shim".
- Both have an older sister who wears their hair in pigtails, Mahoro Shimano and Hana Shimura respectively.
- Both dreamed of becoming a hero.
- Both have a family member that hated heroes and disapproved of their dream of becoming a hero, Katsuma with his sister Mahoro and Tenko with his father Kotaro.
- Both have at least one dead family member.
- Both were actively targeted by a villain with an All For One Quirk. Katsuma was targeted by Nine for his Cell Activation Quirk while Tenko was targeted by All For One himself to be groomed into becoming a villain as an insult to Nana Shimura.
- (Katsuma to Izuku) "I'll get stronger! I promise you I will! I'll get strong so that way I can protect Daddy and Mahoro! And then I'll be a cool hero like you and Bakugo! One who always wins no matter the odds!"