On your mouth is a stamp that you strike and beat to create a beat! The stamp looks like a little drum, but you're really just hitting your mouth for fun (the stamp was an excuse to make it seem normal).
As you strike and beat your own mouth, こういち comes dancing into your bathroom. "Hey now, this looks fun, can I beat your mouth up a bit too?" he asks.
Currently there are no similar looking kanji which exist in the WaniKani learning system