Keroyon was a member of Kenji's Youth Group who, along with Konchi, declined Kenji's invitation in rebelling against Friend and taking back the symbol. Because of this, he survives the events of The Bloody New Year's Eve of 2000 and wasn't branded a terrorist amongst Kenji's group. Between 2000 and 2015 he established a food truck called Frogs Japanese Soba with his son being the only employee. It's also revealed that he and his wife split during this time. In 2015, Keroyon and his son is seen traveling across New Mexico, U.S.A, in their food truck in hopes of setting up shop somewhere that makes his soba stand out from the local cuisine scene. Keroyon and his son decides to set up shop in a small town that is revealed to be a victim of the new virus that Dr. Yamane engineered, where he learns that Kiriko might be in the country from a child who took her vaccine and survived. He then pursues clues of her whereabouts to Missouri, then to Lake Michigan and eventually finds her in a drug factory which is on fire. After rescuing Kiriko from the fire, Friend's gas mask wearing salesmen finds them and takes Kiriko's vaccines and research in exchange for letting Kiriko and Keroyon go.
Between 2015 and the Third Year of Friend, he establishes The Frog Empire, a makeshift fortress albeit with considerable size. The Frog Empire is the base of operation for Keroyon, his son and Kirko, although presumably it was established to give Kiriko the resources and security she needs to develop the vaccines for the evolved form of Dr. Yamane's virus, the virus which Friend intends to use to end the world. He along with Maruo are overseers to Kiriko's human trial on her vaccine. After the vaccine is proven to be successful, he replaces The Frog Empire flag with Otcho's symbol, showing his resolve in taking back the symbol along with his friends. Within the fortress, he also grows a vast field of buckwheat, showing his passion for soba noodles.
He, along with Maruo, Kiriko and his son, goes on to distribute the new vaccine. When infiltrating Tokyo with Sadakiyo's help, he takes leadership in climbing the wall and pulling everyone up. When he and Maruo finds Kenji, Maruo hands him the crates of vaccines and tells him to distribute them and to warn the citizens of Tokyo about what's to come.
Keroyon is seen together with the remaining members of Kenji's Youth Group at Kenji's concert, reunited at last.
Presumably in 1999, he has a relationship with a woman despite already being married and having a child. This causes tension between him and his wife, and coupled with his failing business, leads to Keroyon ignoring Kenji's invitations, both times not even bothered taking a look at it. Ever since the Bloody New Year's Eve of 2000, Keroyon has felt guilty over ignoring Kenji. Over the years since, he constantly laments about how he's "the lowest" and his life "is just running away", much to his son's dismay. When he finds the drug factory Kiriko is in is on fire, he selflessly rushes in to save her and despite suffering major burns to his entire back, manages to find redemption.
As a child, he often helps out with his family business. He, Konchi, Mon-chan are witnesses in Donkey's Science Lab incident, and is one of the participants of the Hanging Hill courage test in 1971. He, along with Kenji's youth group, defended their secret base against Yanbo and Mabo's raid.
Presumably his real name is Keitaro Fukuda (福田啓太郎).