Kimihiko Nikaido is a character who appears in the film L: change the WorLd. He is a man of science, holding the position of a professor at the Infectious Disease Center of Asia. His physical appearance is that of a tall, tanned man with a slender build and short, dark hair that is slightly graying. He also sports a mustache and a beard on his jawline. In his professional setting, he is typically seen wearing a doctor's coat or a chemical protective suit.

Professor Nikaido's personality is defined by a powerful combination of profound intelligence and deep-seated compassion. He is a dedicated researcher, capable of formulating an antidote to a deadly, mutated virus in a remarkably short period. However, his true moral character shines when his work is threatened. He places the greater good of humanity above his own life, demonstrating an unwavering resolve to prevent a deadly virus from being used as a weapon. He is so committed to this principle that he is willing to die rather than surrender the antidote to a bio-terrorist organization. This also highlights his strong opposition to indiscriminate killing, as he explicitly condemns the terrorists' plans by stating they are no better than Kira.

In his personal life, Kimihiko is a widower and a devoted father to his young daughter, Maki Nikaido. His relationship with Maki is exceptionally close, built on love and responsibility. He is not merely a parent but also her primary caregiver and teacher, as he is shown cooking, cleaning, and homeschooling her. He is attentive to her medical needs, as Maki requires daily injections and temperature monitoring to manage a health condition. His love for her is evident; he sees his late wife in her and treasures that resemblance. The homework assignments he creates for her are complicated puzzles, which he uses to instill a love of challenge in her, often telling her that if there are difficulties, it means the challenge is worth taking on.

Kimihiko Nikaido's role in the story is that of a catalyst. He develops an antidote for a highly contagious and lethal virus that has been engineered by a bio-terrorist group called Blue Ship. When his colleague, Dr. Kimiko Kujo, reveals herself to be a member of this group and threatens his daughter to obtain the antidote, he is forced into a desperate situation. To protect his creation and the world, he tricks the terrorists by destroying the research data and incinerating the only sample of the antidote in front of them. When they attempt to use Maki as a hostage, he makes the ultimate sacrifice, injecting himself with the deadly virus rather than surrendering.

His key relationships, besides with his daughter, are primarily professional. He works closely with Dr. Kimiko Kujo, trusting her as a colleague, which makes her betrayal a deep shock. He also co-wrote a book on infectious diseases with a colleague named Koichi Matsuda, who later plays a role in helping to recreate the antidote after Kimihiko's death.

In terms of development, Kimihiko's character is static in his morality but dynamic in his actions under pressure. He begins as a dedicated scientist and father who believes in the goodness of people, but he quickly adapts to the reality of the threat against him. His key moment of development is his active choice to die on his own terms to uphold his ethical beliefs. This act is not a sign of weakness but a final, decisive action that sets the entire subsequent plot into motion. Regarding notable abilities, his primary skill is his genius-level intellect in the field of virology and immunology. He possesses the scientific knowledge and capability to rapidly analyze and counter a sophisticated bio-weapon, an ability that makes him a target for the terrorists and a potential savior for humanity.