"不" (fu, etc). It is a meaning no, not, etc. It is used as "un-" prefix.
This character is written by Kanzi, Regular script.
"不" originated as a pictographic character which depicted the calyx of a flower.
It expresses various meanings with the phrase.
"不変"(fu henn) is a meaning immutability, eternity, etc.
"不思議"(fu shi gi) is a meaning wonder, strange, etc.
I guess that it is suitable to express Nature.