A friend of Koiwai and Yotsuba, having known Koiwai since they were childhood classmates. Standing 210 centimeters (6 ft 11 in) tall, he dwarfs the other characters, especially Yotsuba. He is always called by the nickname "Jumbo," and when Fuka learns his real name she calls it "common." He works at his father's flower shop, as Yotsuba discovers when she meets him there. He helps the Koiwais move in and frequently visits their house, where he is more or less treated as a member of the family by Yotsuba.
Jumbo has a dry sense of humor and is prone to deadpan quips, which sometimes confuse members of the Ayase family. At the same time, he is impulsive, and sometimes spontaneously organizes activities for the younger children, such as fishing and star-gazing. As he does this, he enters a bizarre one-sided rivalry with Miura, including spending a single day in Hawai'i after he learns that she will be going abroad. He develops a deep infatuation with Asagi when he first meets her, but is too nervous around women to directly act on it. Instead, he tries to use Yotsuba's friendship with Asagi as a way of getting close to her, though his plans usually backfire.