Cullen Calix is a supporting character from the anime special Pokémon: Mewtwo Returns. He is a Pokémon naturalist who specializes in medicine and scientific research. His Japanese name, Penicillina, is a reference to penicillin, which hints at his expertise in the field of healing and remedies.

Cullen's primary motivation is his passion for scientific discovery. He travels to Mount Quena for the specific purpose of conducting research on Clarity Lake, a location famous for the alleged medicinal properties of its pure spring water. His personality is defined by a genuine, if initially single-minded, scientific curiosity. He is portrayed as a knowledgeable and professional researcher, and he arrives at the area in the company of a young woman named Domino, who he believes is an employee of the Pokémon Institute. Cullen is unaware that Domino is actually a secret agent for Team Rocket, indicating that he is trusting and not naturally suspicious of others.

In the story, Cullen serves as a neutral scientific presence who becomes drawn into the central conflict between Mewtwo and Team Rocket. He, along with the local resident Luna Carson, Ash, and his friends, helps to fight against Team Rocket when they threaten Mewtwo and the cloned Pokémon. His role evolves from an observer of the lake's properties to an active participant in its defense. A key moment for his character development occurs when he witnesses the baby Pokémon among the clones, such as a Nidoqueen and a Rhyhorn, and sees Luna’s deep, personal attachment to the area. This experience gives him a deeper respect for Clarity Lake and its natural environment. Consequently, he decides to abandon his plan for cutting-edge research on the lake itself, fearing that his work might jeopardize the fragile ecosystem. Despite this, his scientist nature persists, and he still takes a small sample of the spring water, hoping to develop a synthetic version of its healing properties in a laboratory, which would allow him to continue his research without harming the original source.

His key relationships include his escort of Domino, which is built on a professional deception, and his alliance with Luna Carson. By the end of the events, he finds common ground with Luna, deciding to stay with her as they share many interests beyond just research. Despite his sincere change of heart and his resolution to pursue a more ethical form of research, Mewtwo, still fearing that any human ambition poses a threat to the sanctuary, ultimately wipes Cullen's memory of the day's events. Regarding notable abilities, Cullen does not possess any supernatural powers or Pokémon training skills. His abilities are entirely intellectual, rooted in his expertise as a medical researcher and Pokémon naturalist.