Mikasa shook her head at his question. They didn't know. Eren only sort of knew his own powers. He knew that if he bit his hand, he could transform, but he also knew he needed to have some determining purpose. But, from what she could understand, Eren was much different than the other Titan shifters out there. Even in what material from her source she had found, they were still uncovering their secrets.
"We still don't know," remarked Mikasa softly. "I think...it may be more at will than not. We've done a few experiments with Eren, mostly done by Captain Levi and Captain Zoe. They've determined so far that for Eren to transform, he needs motivation, a single reason to use his powers. Once he has that, he can transform. Something even as simple as wanting to pick up a spoon is enough."
She paused, the dislike of the creatures written all over her features. Her eyes were darker, narrowed, and her lips drawn together in a frown. Her hand still remained clenched at her fist. "Even where they come from or what they are, we don't know. So much we don't know yet. We're more prepared than we were five years ago when they first appeared after a hundred years of nothing, but we still don't know so much. Captain Hanji Zoe captured two Titans similar to what you just did here to learn more, but the Titan-shifters are new."
She looked back at the two Titans before them. "I imagine that if Eren hadn't been one, we wouldn't know anything. I don't think we were meant to know, some how."