I hate spoiler previews, as I never watch them... but people keep talking about them.
Had to watch that one too.

So Eren would not be the only one to be able to transform and be able to go back to human form. But he's only a rookie at it... since it seems that other one might have more control. The problem being she's against humanity. Or should I say against that group of humans.

I guess everyone felt that when Eren got his power in the show, the Titans as they are would not be enough to have an eventful story. It's only normal we get something else to counter that new power.

Now we're back to who controls that Titan power, who conducts the experiments creating such monster, and why.
It could really be the inner walls humans, or another group from the outside.

BTW, having controllable Titans would explain how it is possible to build 40+ walls quickly, even under attack... The only problem remains information control. I wonder how it is possible nobody from the outer walls remembers how the walls came to be.