Leon McNichol is a senior officer in the Advanced Police force, commonly referred to as the AD Police, in the cyberpunk setting of Bubblegum Crisis Tokyo 2040. He is depicted as a dedicated but often frustrated law enforcer who approaches his duties with a gung-ho attitude and a strong sense of responsibility, viewing himself as a guardian of the public. His personality is marked by great self-confidence and a refusal to back down from a challenge, though he is also prone to frustration, particularly when the vigilante group known as the Knight Sabers consistently intervenes in rogue Boomer incidents before the AD Police can effectively respond. This professional rivalry fuels his initial strong dislike for the Knight Sabers and their methods. Despite this antagonism, Leon is motivated by a sincere desire to protect the city and its people, and he harbors a deep abhorrence for police corruption and apathy, which occasionally leads to friction with his superiors.

In the story, Leon serves as a primary point of view within the AD Police, frequently clashing with and later reluctantly cooperating with the Knight Sabers. His most significant relationship develops with Priss Asagiri, the lead singer of an underground rock band and a core member of the Knight Sabers. Leon initially pursues her with romantic intentions, and despite her initial mockery and distrust of law enforcement, the two share numerous dangerous encounters that gradually shift their dynamic. Over the course of the series, Leon learns that Priss is a Knight Saber, yet they eventually confess their feelings for each other, forming a romantic side story that intertwines with the main conflict. He also maintains a close professional partnership with his colleague Daley Wong and is playfully teased by AD Police dispatcher Nene Romanova, who affectionately calls him Leon-Poo.

As a character, Leon undergoes notable development from a headstrong officer frustrated by vigilante interference to someone who actively supports the Knight Sabers in their final battles. After Tokyo is overrun by rogue Boomers, he shifts his focus from institutional law enforcement to helping civilians evacuate while the Knight Sabers confront the ultimate threat. In terms of abilities, Leon is recognized as one of the highest ranking and most skilled officers on the AD Police force, particularly as a pilot of powered armor. He is also an experienced gunner and close-quarters combatant, often seen wearing a jacket and fingerless gloves that reflect his practical, hands-on approach to police work. His physical appearance includes short gray hair and red eyes, and his overall demeanor combines toughness with a genuine, if sometimes impulsive, commitment to doing what he believes is right.