A friend of Koiwai and Yotsuba, Jumbo has known Koiwai since they were childhood classmates. Standing at an impressive 210 centimeters (6 ft 11 in), he towers over the other characters, especially Yotsuba. He is always referred to by his nickname "Jumbo," and when Fuka learns his real name, she finds it rather ordinary. Jumbo works at his father's flower shop, a fact Yotsuba discovers when she encounters him there. He assists the Koiwais with their move and often visits their home, where he is treated like a member of the family by Yotsuba.
Jumbo possesses a dry sense of humor and tends to make deadpan remarks that sometimes confuse the Ayase family members. Despite this, he is impulsive and occasionally organizes spontaneous activities for the younger children, such as fishing trips and stargazing sessions. In doing so, he develops a peculiar one-sided rivalry with Miura, even going so far as to spend a day in Hawaii after learning she would be traveling abroad. Upon meeting Asagi, he becomes deeply infatuated with her but is too nervous around women to act on his feelings directly. Instead, he tries to use Yotsuba's friendship with Asagi as a means to get closer to her, though his plans often backfire.
