A/N: Please know that this is heavily based off stories that were told to me - including the shifting labels - and doesn't reflect my own personal experiences in any way. If anything is offensive or harmful in any way, do let me know.

Title is taken from Centuries by Fall Out Boy.

It had started with a niggling discomfort under his skin. Whenever his parents said boy or introduced him to people as my son, something had rankled deep inside. He didn't have the words to explain it to himself, much less to other people, so he grit his teeth and tried to grin and bear it, ignoring the wrongness inherent in the words. When he spoke about his desire to be a police officer and protect people, everyone nodded and congratulated the choice. Perfect for a boy like him. A good choice for a man. Something about the words didn't sit right in his chest, but he ignored it.

When he was seventeen, he encountered the word non-binary and something about it was incredibly appealing. They. The pronoun didn't fit exactly right, almost like a shirt that was cut wrong, but it worked better than he and his. The idea that the lines that strictly divided boy from girl could be so easily defeated made them laugh with pure joy and they embraced it. Their ambitions didn't change, but suddenly there was a non-spoken assumption that made them want to pull their hair out. Maybe something more private? Less out of the public eye, one well-meaning great-aunt offered and it was words and stares like those that fanned the flames of ambition, fueling the countless A's needed for the police academy.

Eventually, though, it didn't fit anymore and seeing the little non-binary flag pin that had been acquired at a pride parade was frustrating. It was a good label, and necessary, but it no longer encompassed the right identity. Every time they introduced themselves it no longer felt good. One of their neighbours who lent them books and gave countless hours of good advice smiled and said, if it doesn't feel good, you need to find something else, and so, they started looking again.

When they hit on the word transgender it was like it had been staring them in the face all along. This was what had been missing. She flipped through websites and books, spent more hours talking to her neighbour, more and more excited. It was freeing, finding the right words to finally explain it all. Not a boy, a girl. She, her, hers. It was right and good and she sat for several minutes testing the words in her mouth and then thinking of a name. Her family accepted it as easily as they had the declaration of non-binary and she kept moving towards becoming a police officer, the goal that hadn't changed even as everything else had.

The academy wasn't half as accommodating and from the moment she laid eyes on the director, she knew. Sure enough, she was assigned to bunk in the male dormitory next to someone named Jake Peralta. He was already there when she arrived, a wide smile affixed to his face and his suitcase half un-packed.

"Hi, I'm Jake. What's your name?" he asked, sticking out a hand.

She took a deep breath and took his hand. Something about the way he carried himself exuded trust and calm and she hoped she wasn't making the biggest mistake of her non-existent career. "Rosa."

"Cool." Jake's smile didn't waver and he shook her hand. "Hope you don't mind sharing a room with me for the next little while."

"I'm a girl," Rosa snapped, half-turning so he couldn't see her face. Had he mis-heard or was he playing a cruel joke like the other kids in high school had? She held her breath, hoping he wouldn't go running to the director about it.

Jake shrugged, his smile fading in the face of her sudden anger. "Okay," he said. "I won't tell anyone."

Rosa looked at him, relief making the walls between her emotions crumble. "Promise me," she intoned.

"On what?" Jake asked, taking a tiny step back. "My life? My mother? Gina?"

Rosa paused, a flash of inspiration hitting her as she took another look at his skinny arms. "On a hundred push-ups," she said firmly.

"A hundred?" Jake asked.

"A hundred."

He shrugged. "I told you I won't tell," he said seriously, "and I won't. Not ever." He grinned. "But I definitely won't tell for a hundred push-ups."

It becomes their inside joke, a way of telling the other to trust them implicitly. They share secrets, even Jake, who most people assume has none, and Rosa knows she can trust him. With her life, and with herself.