Leon is Lin Grace's knife wielding bodyguard. He often appears out of nowhere pressing a knife to Densuke's throat.
He was introduced in Volume 1, holding a knife to Densuke's neck thinking he was trying to pull a weapon on Grace, when he was just asking about Chiharu's missing wallet.
In Volume 3 it is was revealed that it was not his job to protect Grace from those who would do her harm, but rather to protect those who would do her harm from Grace.
He only appears in the Manga.