I assume you are talking about constant values in expressions. Any letter (including Greek ones) can be used to represent a constatnt, but the most common one is k.
There are, however, some constants which are associated with other symbols:
Ratio of circumference of circle to diameter = pi
Speed of light in vacuum: c
Gravitational attraction: G (not to be confused with g, in the context of the earth's gravity)
Avogardo's constant: NA
A constant like "c" equals the speed of light in E=mc2
It is often represented by c for constant or k for its phonetic equivalent. But as long as you declare it as a constant, any symbol will do.
a symbol representing courage is a black guy
The symbol of Gemini
Representing a number in decimal form does not change its value. Its value remains at 79 however it it written.
A symbol representing a process
The symbol that represents change is delta. (It looks like a little triangle before the unit) ..../\..... ../....\..... /____\....
H2O(g) is the best way of representing water vapour, the 'g' within the brackets representing 'gas'. There is no specific symbol.
To the best of my knowledge there is no symbol for change but the closest would probably be the Dharma wheel which symbolises the cycle of birth death and rebirth. The same thing could be said of growing up but I suppose the closest symbol would be the white lotus which is a symbol representing the attainment of Enlightenment.
The symbol for neon is Ne and it is the short form for representing the element neon.