User blog comment:Xxkiiroxx/"Marco is Alive" Theory/@comment-173.18.133.12-20141004080002/@comment-5433366-20141004081815

Let me explain this for you.

It's called the Female Titan arc. The Female Titan's host according to Erwin escaped the Corps' wrath by prompting Titans to eat itself. The host is nowhere to be seen, but a hooded figure (whom Gunther first thinks is Levi) kills Gunther. Erwin explains that, if the Titan could have prepared a Corps uniform and 3DMG beforehand, it could disguise itself as a member and transform once again. Shortly after murdering Gunther, the hooded figure, just like Erwin explained, transforms. It is not an accomplice, it is Annie. The only reason the physical details of the hooded figure are not stressed is because Isayama (and the animators) wanted it to remain mysterious.