Jemma didn't remember the first time she noticed it. He'd say something a bit sharper than he used to; he'd snap at something that used to make him laugh. Maybe it happened after she nearly died and Ward saved her. Maybe it happened after the mission he went on with Ward that had no extraction plan.
He was coarser than he used to be. Not by a lot. Just a few edgy jokes here and there, a few rolled eyes and sarcastic quips. He cut corners where he used to be meticulous; he blew off paperwork that he used to be conscientious about.
Perhaps he wanted to be like Ward; perhaps he wanted to be tough, strong, proud, and oh so sure of himself. But Jemma missed his soft edges, his humble sweetness, his fumbling, unsure attempts to make her feel better whenever he could.
It all came to a head when they were going to save Coulson and Fitz didn't seem to care if super-soldiers were injured or killed in their attempt to rescue their C.O. She felt odd being the one to argue for caution and compassion, the one to slow things down and think things through, the one to carefully weigh the pros and cons.
She had done this to him. She dragged him onto the "Bus", using an ingrained friendship as a lure. She felt the guilt weigh upon her heart like an anchor; she felt responsible for his metamorphosis. And she knew, without a doubt, that it wasn't over and it wasn't going to stop. He was changing day by day, ever so slowly into someone that she couldn't recognize. He wore the face of a friend, but he wasn't her Fitz anymore. The darkness had begun to slowly seep in through the cracks, had begun to muddy the waters. And she didn't know how to turn back the clock.