Everyone on Survey scout doesn't know only Erwin know Eren was younger brother of Humanity strongest and strongest soldier. Eren didn't tell everyone even his friends but not really friend (Armin,Jean, Connie,Sasha, Christa and Ymir) about his last name because Eren was an Ackerman and people always scared about Ackerman. One day,Eren was eating along his friends until Levi and Mikasa called Eren "chibi"to go training with them. Everyone was shocked about the nickname.