You become a real person from the moment you are born. Some would say from the moment you are conceived.
Mermaids are not real. They do not exist and are from stories. A person can not become a mermaid.
Make a video about yourself and show you the real person and you will become famous. 
Show me the money first........
In real life, there are no such things as were-cats. So it is impossible.
It is about what it means to become a well-balanced person and a real man.
the REAL person to discover Americas mothers middle name was America
can vampires become real
All the property, real and personal, that a person owned at the time of their death will become part of their estate.
All 3 creatures you ask about are not real and you can not become any one of them. Be happy as a person.
Fairies like "Tinker Bell" are part of "fairy stories". Fairy Stories are fictional creations and the characters are not real. Therefore I am sorry to say that "Fairies" are not real, they are imaginary beings and it is impossible for a real person to become one.
No, fictional characters can not become real.
There are no spells or anything else that can turn a person into a mermaid. Spells are not real. Mermaids are not real and these things only exist in films and movies.