Kei, a medical student, participates in scientific expeditions exploring the human body. He operates externally alongside Dr. Brain, assisting while others miniaturize for internal exploration. Kei diligently monitors Phoebe's condition, administering necessary treatments like an anthelmintic pill to eradicate a hookworm parasite discovered in her small intestine. He demonstrates concern for her well-being, yet sometimes chooses to withhold medical information, believing certain details could unsettle her or exceed her grasp of the science. Kei interacts directly with Phoebe, attempting to educate her about bodily functions; she often reacts with disbelief or finds the information distasteful. His role centers on coordinating between the external environment and the internal explorers, using his medical expertise to address health challenges encountered during their journey through bodily systems.