This was an interesting symbol to choose for an internet explanation. It took me a while to realise that the rectangle is actually the symbol that I'm supposed to see, rather than a missing glyph.
2
--------+
1 \ /
\ ---- /
3
Image search for "kuchi-no kanji + shodou (calligraphy)":
https://www.google.com/search?q=%E5%8F%A3%E3%81%AE%E6%BC%A2%...
It seems there are two schools on the relationship between stroke 2 or 3: 2 can extend downward past 3, or 3 can underscore 2.
Maybe it is a missing character after all :) (Oh no, I've been fooled by chrome/freetype/something!)
(I was expecting it to be a rant about insufficient unicode usage.)
That said, I expected something Japanese related, given that it's Candy Japan, but you'd think common fonts would have different looking characters for “no glyph” and the kanji for mouth.
Ha, for me, they look quite different in the font used for editing, but not so much in the rendered comment other than a small difference in size.
It's not clear whether or not that naming for that character originated in Japan.