There are several people who either come into contact with or indirectly affect the main characters of the story.
Hanabata Shinpei. He meets the Cell Four guys at the JSDF garrison. He and his childhood friend Kasahara Yuuji are looking for weapons to protect themselves on their quest to find a doctor for his son Riku.
Kasahara Yuuji. He too meets Cell Four at the JSDF garrison. A loyal friend, he is traveling with Hanabata to find a doctor for his infected son. Kasahara knows there is no hope or cure for the child, but follows his friend anyway.
Dr. Yukko Sakagawa. This doctor contacts the Shouran Academy from her lab on the Sea Firefly. She asks the boys to carry out her mission of bringing her a newly infected F-Virus carrier so she can make a radical cure for the M-Virus.