You were born without knowing who your real parents were so you lived in an orphanage. But all anyone ever told you is your parents were scouts and they died on a mission to kill some titans. When you got older you joined the scouts to honor your parents. But you grew cold, feeling like nobody cared about you but that only drove you to be the best in the scout regiment. So eventually you made it second in line to Levi Ackerman himself and now you are part of the Levi squad.