Now here is a theory that may get your head cogs turning
Eren accidentally brainwashed Mikasa into being the perfect fighter with the Coordinate. A supposition (by Commander Erwin to Dot Pixis):
• Eren can (as Coordinate) control Titans.
• Humans can become Titans (evidence: Connie's mom)
Therefore: Eren (as Coordinate) may be able to control Humans.
• Evidence: in two seperate cases, we have seen Mikasa act to protect her life when she was but a moment earlier unable to:
? At age eight, Mikasa is ordered by Eren to fight to save him from being strangled by her kidnapper. This scene is accompanied by a flash of green lightning around Mikasa's body and a shot of what appears to be Mikasa's brain. Then, Mikasa engages the kidnapper with such ferocity that she, an eight year old, crushs the handle of her knife.
? Mikasa, in the battle of Trost, has just run out of gas for her 3DMG and has given up hope, declaring she has lived a good life and seems ready to die to the hands of a nearby Titan. Then, we get another shot of her brain, and suddenly she dodges the Titan's swipes, all the while monologuing mentally at how she is shocked that she isn't letting herself be killed.
• Events explained by conclusion:
? The presence of lightning: almost regularly, whenever we see a Titan-Shifter transform, we see a flash of orange lightning. We've seen it with Bertolt at the gates, Eren when he emerged from the Titan for the first time, and when Annie transformed. However, we have also seen colored lightning before: Mikasa was surrounded by green-lightning when Eren ordered her to fight her kidnapper.