Ectoplasm is a pro hero who serves as a member of the faculty at U.A. High School, where he is primarily responsible for teaching mathematics. At forty-one years old, he is a seasoned educator and hero known for his unique and somewhat intimidating appearance. His most distinctive physical trait is a wide mouth that appears to be split from ear to ear, revealing a full set of teeth. He typically wears a white high-collared trench coat and a dark mask or cowl that covers much of his head, which adds to his somber visual presence. He stands 180 centimeters tall. A key element of his backstory is the loss of both his legs in a past battle with a villain. Rather than retiring, he chose to continue his career as a hero and now uses specially made prosthetic legs, which are integrated into his hero costume for combat.

In terms of personality, Ectoplasm is known for being a steadfast, unrelenting, and disciplined individual. He has a strict, almost militaristic approach to teaching, employing what is described as a Spartan-style method to push his students to their limits. He firmly believes in putting students under pressure to see them shine and has openly warned them that he would do his best to defeat them during exams. Despite his fearsome looks and rigid demeanor, he is deeply respected by his students and is considered a reliable and responsible teacher who genuinely wants the best for them. He has a unique speech pattern, often speaking with everything in katakana except for kanji characters, which makes his way of talking very recognizable. Outside of his hero duties, he has a notable fondness for karaoke, which is more than a simple hobby as it directly relates to his Quirk.

Ectoplasm's central role in the story is as an instructor at U.A. High School. He is heavily involved in the students' training and development. During the final exams of the first year, he was the designated opponent for the team of Tsuyu Asui and Fumikage Tokoyami, a match chosen specifically because his abilities could exploit the weaknesses of Tokoyami's Dark Shadow. During the training camp arc, he helped Class 1-A develop their ultimate moves by creating numerous clones for each student to spar against in Gym Gamma, demonstrating his ability to single-handedly conduct high-level training exercises. He also appears in later chapters as part of the faculty discussions, showing his continued concern for the students' well-being even after major conflicts, such as when he noted that some students might still want media attention while others do not.

He maintains professional relationships with the other U.A. faculty members, including Shota Aizawa (Eraser Head), Hizashi Yamada (Present Mic), and Toshinori Yagi (All Might). His primary relationship with the students is that of a strict but respected authority figure who challenges them to grow. His bond with his students is evident in small but meaningful moments, such as when he helped Eri cut her horn so she could contribute her quirk to aid the heroes in a later battle against the villains, showing he is willing to support unconventional but rational plans put forth by younger individuals.

Regarding his development, Ectoplasm's history is marked by a significant physical trauma—losing his legs—which makes his current status as an active pro hero a testament to his resilience. He is often cited as an example of an indomitable man who refused to be defeated by his injuries, earning him deep respect. His fighting style has evolved to accommodate his prosthetics; he specializes in powerful kick-based melee combat and relies less on hand-to-hand techniques, a practical adaptation to his condition. He has also shown personal growth from his own student days, where he recalls sometimes oversleeping and once even using a clone to attend class to achieve perfect attendance, an act he now views with some shame, which informs his high standards for honesty and discipline in his own students.

The notable ability of Ectoplasm is his Quirk, named Clones. This Quirk allows him to expel a smoke-like substance, or ectoplasm, from his mouth and shape it into physical duplicates of himself. The clones act as extensions of his will and can be created at various locations within range. Under normal circumstances, he can maintain approximately thirty clones at once. However, the output and number of his clones are directly tied to the condition of his throat. By warming up his voice, for instance by singing two or three songs at karaoke, he can increase his maximum output to thirty-six clones. The clones themselves have a decent level of durability and can be dispelled by a strong enough blow, though Ectoplasm can also dispel them at will. His ultimate technique is called Forced Internment Giant Bites. This move involves creating one gigantic clone, which can be as large as a building, that swallows the target whole. Once inside, the target is trapped within the clone's body. The primary drawback of this powerful technique is that while the giant clone is active, Ectoplasm cannot create any of his regular-sized clones, leaving him vulnerable in terms of his usual tactical flexibility.