The answer is
I (the first-person pronoun)
But I'm having some trouble with the reasoning for almost all of the lines, so I'll make it a community wiki and maybe others can offer some insight.
For some who believe, it is a ghost.
In mathematics $i$ is the notation for an imaginary number.
Depending on others, it may boast.
Boastful statements might well contain the word "I".
Its elder twin is quite selfish.
Its elder twin is the Latin word "ego", a term which connotes selfishness.
Even the dead feel bad, especially Elvis.
Elvis had a song titled "I feel so bad".
Today, its uses are vast.
We use the word for all kinds of statements and speech acts.
One can decide its caste.
A play on "case". Letters can be lowercase or uppercase. Also, the first-person pronoun has different cases: subject ("I"), object ("me"), possessive ("my").
One uses it all the time.
It's probably the most frequently used pronoun.
Doesn't cost one a dime.
Speech is free.
It's only 53. So, still a long way to go.
"I" is the symbol for Iodine, which has atomic number 53
Begin to worry if one doesn't know.
If one doesn't know about the pronoun "I", it's unclear how one could express oneself, let alone understand others. This would be cause to worry (???). OP's edit: if you don't know about yourself, you are in trouble.
After all, it's a masterpiece for a few..
Our uses of "I" are kind of like tiny self-portraits (???). (OP's edit: use of i(the imaginary number) in Euler's famous equation.)
No, it hasn't got anything with dew.
A pun on "I do" and maybe also "deux" (since "i" is the Roman numeral for one).
There's a reason this riddle is written in third person.
Because otherwise you'd have written the answer ("I") all over the puzzle.