Cullen Callix is a supporting character from the Pokémon television special Mewtwo Returns. He is a Pokémon naturalist with a specialization in medicine who travels to the remote Mount Quena to conduct research on the environment of Clarity Lake and its alleged medicinal properties. In his role as a researcher, he escorts a woman named Domino, believing her to be an employee of the Pokémon Institute, and remains unaware of her true affiliations with Team Rocket. His professional focus and initial motivations are driven by scientific inquiry, specifically the desire to study and potentially utilize the lake's unique healing qualities.

Throughout the story, Cullen’s character is defined by his interactions with the other residents of Mount Quena. After Team Rocket's actions force a confrontation, he joins Ash and his friends, the local researcher Luna Carson, and the legendary Pokémon Mewtwo in fighting against the villainous organization. It is during this conflict that Cullen’s perspective begins to shift. Witnessing the presence of young, cloned Pokémon like a baby Nidoqueen and Rhyhorn living in the area, and seeing Luna’s deep personal attachment to the lake, he develops a profound respect for the natural balance of Clarity Lake. As a result of this experience, he makes the deliberate decision to abandon his cutting-edge research on the lake's direct healing properties out of fear that his work could jeopardize its fragile environment. Instead, he collects a single sample of the spring water with the hope of eventually developing a synthetic version, a compromise that would allow him to pursue his medicinal goals without disturbing the natural site. Despite this personal resolution and his demonstrated willingness to protect the area, Mewtwo ultimately erases Cullen’s memory of the specific events, still fearing that his underlying ambitions could pose a future threat.

Following the incident, Cullen decides to remain at Mount Quena with Luna Carson, recognizing that the two of them share a great deal in common beyond their academic pursuits in research. His notable abilities lie in his expertise as a medical specialist and researcher, with his Japanese name, Penicillina, being a direct reference to his skills in the healing arts. In terms of development, Cullen begins as a scientist focused on extraction and study, but his journey leads him to value preservation and coexistence, ultimately finding a new purpose in the very place he came to analyze.