This is a man who fights ...
Each line in the verse describes a word that begins with Le– and ends in one of the letters from s to y, so the word at the blank must be Lex. Lex Luthor is a supervillain who fights Superman. Superman was born on Krypton as Kal-El, and that rhymes with "tell". I've got all tags covered, I hope.
How do you do, my name's Gavroche,
Gavroche is the titular street orphan in Victor Hugo's Les Misérables.
whispering words of wisdom, see?
Let it be.
Bucharest coins are nothing posh,
The currency of Romania is the Romanian Leu.
neither is neighbouring currency.
The currency of Bulgaria is the Bulgarian Lev.
This is the man that wrote the song "Charmaine",
Lew Pollack and Ernö Rapée wrote that song in 1926.
this is a man who fights (???).
Lex Luthor is a supervillain and Superman's enemy.
In Spanish courts this word is often named.
Ley is Spanish for law.