Daimaru Sumiyoshi
About
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.