Daimaru Sumiyoshi, usually referred to by his surname, is one of the three residents living in the apartment next to Excel and Hyatt, the other two being Norikuni Iwata and Toru Watanabe. The three are almost constantly together, appearing throughout the series as one of the many side stories going on behind the main storyline. The ever-silent counterbalance to Iwata's antics and Watanabe's uptight normality, Sumiyoshi has a sensical, but fun-loving personality, and often acts as something of a tiebreaker when disputes break out between the two of them.

Most noticable about Sumiyoshi is the fact that he never speaks aloud, instead conferring all of his thoughts through standalone text within the panel or on-screen. This is used as a running gag throughout the series, and is even referenced on occasion by other characters. (Ex. Iwata complaining that he can't read it when he writes it so fast)

Sumiyoshi's appearance is that of an overweight, average-clothed young man with a very wide face, glasses that hide his pupils, and a wide, perpetually closed mouth. The facts that his expression almost never changes and his eyes are rarely, if ever, shown are both running gags as well.