Formal:
* S'appelle Leonie. (This is the most common and neutral way to say it.)
* Elle s'appelle Leonie. (This emphasizes that the subject is female.)
* On l'appelle Leonie. (This is less common and implies that people refer to her as Leonie, perhaps her nickname.)
Informal:
* C'est Leonie. (This is a very simple way to say it, often used in casual conversation.)
Example:
* La petite fille s'appelle Leonie. (The little girl is called Leonie.)
* C'est mon amie, Leonie. (This is my friend, Leonie.)
* On l'appelle Leonie, mais son vrai prénom est Léonie. (We call her Leonie, but her real name is Léonie.)
Remember that "Leonie" is spelled with an "e" at the end in French.