Hannah's a member of the 104th Trainees Squad, introduced early on when she's spotted walking through Trost District with Eren and Franz. There's a running joke where Eren immediately assumes she and Franz are a couple—both of them get flustered and deny it pretty hard.
She first shows up in the manga at Chapter 3 ("The Night Of The Disbanding") and makes her anime debut in Episode 3 ("Shining Dimly in the Midst of Despair").
Content compiled by AnimeList.moe from publicly available sources.
