Erwin isn't your biological brother. He found you when he was about 15, and even tho he had many things to do, as he was cadet back then, he decided to keeo you. Since that time he raises you, but he never lied to you about fact that you're not his biological sibling. You don't care about this anyway, your parents died or didn't want you, so you don't even want to meet them.

You're currently 13 years old and you joined Training Corps last year. Of course Erwin have many duties to funfill as commander of Survey Corp, but he always finds time for his little sibling to help them with ODM gear. "Good... try to hold balance a bit longer..." Erwin says as you struggle to hold proper postion at ODM training gear