Kensuke Aida is a fourteen-year-old student in class 2-A at the municipal school in Tokyo-3 and a close friend of his classmate Shinji Ikari. He is one of the "Other Children," a pool of potential Evangelion pilot candidates secretly assembled by NERV. His background reveals that his mother is deceased, and his father works at a NERV installation, a commonality he shares with many of his classmates.

Kensuke's most defining characteristic is his intense fascination with military paraphernalia and technology. He is a self-described military enthusiast, or otaku, who spends his free time camping alone in the mountains surrounding Tokyo-3 and engaging in simulated war games. He is almost always seen carrying a handheld camcorder, eagerly recording military vehicles, Evangelion units, and any related activity. This passion fuels his primary motivation: a deep and unwavering desire to become an Evangelion pilot himself. He frequently begs Shinji to use his influence with NERV to get him selected, seemingly oblivious to the immense physical and psychological trauma that pilots endure, as witnessed firsthand during battles. Alongside this ambition, he also expresses envy towards Shinji for living with the attractive Misato Katsuragi, on whom he, like his best friend Toji Suzuhara, has a crush.

In the story, Kensuke serves primarily as a friend and classmate to Shinji, representing a normal life that is increasingly overshadowed by the Angel conflicts. His friendship with Shinji solidifies after Shinji saves him and Toji during an Angel attack. Initially hostile towards Shinji for his sister's injury, Toji, often guided by Kensuke's more outspoken enthusiasm, becomes part of Shinji's first real circle of friends. Kensuke's character is typically energetic, self-confident, and sociable, easily approaching others and often acting as a mediator to smooth over uncomfortable situations. His cheerful and somewhat childish demeanor contrasts sharply with Shinji's introversion and anxiety. However, this ebullient exterior can give way to a more selfish and intense side, as seen when his jealousy over not being chosen as a pilot leads him to lash out at a grieving Shinji after Toji is critically injured piloting Evangelion Unit-03.

Kensuke undergoes a significant transformation in the Rebuild of Evangelion film series, particularly in the final installment. Over the fourteen-year time skip, he has matured into an adult and become a foundational member of Village 3, a survivor community. His survival skills and mechanical know-how, born from his military otaku hobbies, make him an invaluable "jack-of-all-trades" who helps keep the community running, fixing equipment and even teaching the village's children. In this new role, he sheds his childish envy to become a mature, grounded, and deeply supportive figure, especially for a traumatized Shinji. He offers Shinji wise counsel, advising him to confront his father rather than run from his pain. His relationship with Asuka Shikinami Langley also evolves dramatically; she lives in his home and calls him by the nickname "Ken-Ken," while he acts as a stable, almost parental figure in her life. While Kensuke has no notable superhuman abilities or piloting skills in the original series, his profound development in the Rebuild films highlights his resourcefulness, resilience, and capacity for growth as a normal human surviving in a broken world.