(A/N) Written between the two Earth-2 episodes, which is why it follows canon but not entirely.
Caitlin was a scientist, and scientists, by and large, didn't dwell in certainties. The grey areas, the maybes, the perhapses, those were where science lived.
But some things had to be certainties.
Constants.
The speed of light, the conservation of energy, the presence of Cisco Ramon in Caitlin Snow's life. Those were the foundations that all the maybes were built upon.
"Cisco, you're coming back," she told him firmly, when he tried to give her the letter for his family.
That was a certainty.
She refused to classify it as anything else.
The same way she'd refused to admit to herself that he could fade away in front of her, vibrating into the space between dimensions, because that could not happen. Not because of the physical impossibility of it (practically everything that happened at Star Labs was, technically speaking, impossible), but because of who it was happening to. Her medicines would still the seizures, Barry would do what needed to be done, and Cisco would be here, with her.
Because Cisco came back.
It was what he did. He came to her apartment with bruises still on his face and the hospital bracelet still on his wrist, and refused to leave until she ate some ice cream, the first thing she'd eaten since the explosion. They hadn't talked for six months after the singularity and now here they were. He'd died and he'd come back. Okay, fine. He hadn't actually died; it was all very timey-wimey.
But.
He was here, with her, that was the point.
Others left, Cisco Ramon never would. Not forever.
He gave her a serious look and a quiet, "Caitlin, please," and she accepted the letter, giving him a look back.
Because he would come back. He would.
After they returned, (like she knew he would) the only thing he said was that they'd seen her doppelganger. "She was … alone," he said. "She was very, very alone."
There was more, she knew. There had to be more. She saw the existence of more in the silences between his words and for the moment (this moment, when there were silences between his words and that crease between his brows and an unwillingness to look at his own face in a mirror) was content to leave them there.
She wondered what he'd seen of his own doppelganger, because she caught him playing with the Vibe goggles more than once, frowning at them, or studying his own hand, thrusting it out ahead of him, fingers splayed as if he were hurling something invisible. When he caught her looking, he dropped his hand, fast, as if he were doing something he shouldn't. The look he gave her was both ashamed and pleading, as if begging her to understand something out of her reach.
The quiet mood he'd brought back with him dissipated like a fog slowly burning off, until he was (mostly) himself again.
"Hey," he said one day. "My letter!"
"I still have it," she said. "I told you I'd keep it."
"Did you read it?"
"Of course not," she said, deeply insulted. "That was private, Cisco, I would never - "
He grinned at her. "Too busy, huh?"
She huffed. Well, yes. Way too busy, what with Geomancer and Velocity-7 and trying to rebuild the speed cannon with Jay. But still.
She went to her desk and pulled it out, slapping it down at his workstation. "I'm not holding this for you again," she warned him. "Find somebody else. The next time you go somewhere that you think this might be required, I'm going with you. You two clearly can't take care of yourselves."
He snorted at her and ripped the letter open. It was two sheets, half of it in Spanish. "Most of it's other junk, never mind about that, but look. Down at the bottom."
The last paragraph. Take care of my friend Caitlin. Make sure she's okay and not alone.
She looked at him. "Cisco," she said helplessly.
He knew she could take care of herself. He knew she could be on her own. He just didn't want her to have to be, even if he could only ask his family to look after her.
"You won't be alone," he told her, and she saw the shadows of what he'd seen beyond the breach moving in his eyes. "You won't ever be alone, Caitlin."
"No," she said. "Of course not."
Someday she would know what he'd seen, and know what her other self had been through, but today, right now, she only knew she wasn't and wouldn't be alone.
It was enough.
FINIS