Sebastian is a mercenary registrar, meaning he will register your character as a mercenary hero. This allows the registered character to act as a hero for other characters.
"Look at your heroes. Now, back at yourself. Now, back to your heroes. Sadly, your heroes are not you, but they could look like you if you sign up for our mercenary services. Anything is possible when you purchase Mercenary Hero Slots from the In-Game Store."