六
One popular theory as to how the roku kanji character came to look this way is that an early form of the character looked like two hands of which the thumbs and index fingers are joined in a circle and the remaining three fingers are pointed downwards: yielding six free fingers with a circle in the middle.
However, earlier evidence suggests that this character was once written as a roof, which was itself a phonetic substitute for a complex character meaning clenched fist, which was an old way of showing six.
The roku character can be read as ROKU or mu- (as in the Chinese counting system within Japanese). As used in: rokugatsu (June), muika (sixth day), and rokkaku (hexagon).