I can learn many things from your replies. Your replies reminded me some interesting comments
on the else thread．It says kanji could be a sign or a symbol.
A native speaker Kohyin-san says "I have been looking at them as symbols."
Oyaji-san says "I think a nice big 「危」 is much more effective than "Abunai" or "Kiken" would be."
I agree with them.
The point is that characters, not only 漢字 but also カタカナ and ひらがな, can be a symbol of something to a native speaker also. This is new to me.
I don't think I can say what I think in my poor English, but let me try.
I understand that Japanese 文字 ( ひらがな、カタカナ 漢字) are Characters rather than symbols because 文字 are elements/components to visualize/spell Japanese words/language. (I'm not sure if this is same thought as yours or not, though.)
On the one hand, every symbol has it's own meaning or role in it, like SS-san shows. Cmmonly, every symbol is independent. There are so many symbols in the world. When you see something, can't find it as a character, you might think it as a symbol(記号)/mark(印) which meanings you don't know yet.
漢字 also has some meanings with each character. Some people say 漢字 is an ideogram. Therefore 危 can be a symbol (or sign, icon) of danger or something. Even Japanese native speakers don't know( or don't care) how to pronounce it, like kohyin-san says on that thread.
My assumption is that 危 became a symbol from an ideogram, in this case. If you can say " 危 symbolizes crisis situations, and it can be used as a symbol ( 「危」は危機的状況を「象徴」し、「記号」としても用いられる)", I think I can understand why some people love to get 漢字 tattoo with in meaningless order. Probably they just try to put some symbols ( not characters) in random order. They might not need pronunciations, just like 危 sign doesn't need it....I might be wrong.
My obsession with "symbol" came from Constitution of Japan, Capter 1- article 1. This is one of my longstanding problems.
Thank You all, for the replies.
(Noob-san, your post helps me lot, thank you.I chose the shortest one.