Colt, who moved in with his brother after breaking his back. So he knows when Stratt took him to study the "black dots", and he knows when Ryland found out his waterless life theory fell on its face, and when he decided to help his students by asking to be let back on the project anyway.
And then Ryland disappears for a couple days, before touching base to let him know he's safe, his clearance just got upgraded, and he's apparently going to be living in a base for the foreseeable future.
And then Grace learns about the coma resistance gene, and maybe that he has it. Confidentiality's a bit lower, because they're recruiting, casting a wide net trying to find qualified volunteers with this gene.
He doesn't know they're only planning enough astrophage for a one-way trip, so he calls up Colt like "hey, if you still have dreams of becoming an astronaut and going to space, now might be your chance — because having this gene can put you ahead of already-trained astronauts."
And so yeah, he jumps at the chance. Stratt knows it's one-way, and doesn't want to separate them. But because Grace was the first to be tested, his identical twin is one of the first viable volunteers: relevant skills so he'll pick it up quickly, an attitude toward danger that's willing, but takes safety precautions seriously.
So she invites him to start training, and when they get enough astronauts with the gene for a crew and backup crew, she's relieved (I actually doubt the crews were all astronauts prior to this; there aren't many people in good health who've been to space for how rare the gene is). She can keep him as a third choice for a third choice — so if (and she really hopes it doesn't come to that) Ryland has to go, they can go together.
When Colt gets the call that Jody's directing a movie, and wants him as her stuntman, well, he's been more or less bumped off Project Hail Mary (Stratt still lets him stick around for availability, he already knows what's up, he might as well keep Grace company), so he heads to Australia.
Then Dubois and Shapiro get blown up, and she tells Grace to look at their other options, but she already knows who it'll be, and she's trying to call Colt back in, but he's not picking up.
And then they get the news — Colt blew himself up, after murdering another stuntman.
That's it — together or not, she needs Grace on that ship, and at least she won't be separating them. He'll wake up long enough to solve the Astrophage problem, and then he can join his brother — he won't have to live a life without his twin.
Grace, of course, objects. Stratt puts him in a coma early.
And then she gets a call on Grace's phone.
"Hey, so I don't know if you've seen the news — but I'm not dead, not a murderer. I kind of need somewhere to lay low until Jody finishes her movie, and then I'll get it all cleared up."
"How would you like to take a trip far away, right about now?"
"...That's a little more permanent than I was thinking."
"We lost Dubois and Shapiro."
"What?! I can't- I'm not a scientist, I can't replace them."
"I don't need you to — Dr. Grace will be the scientist."
"He understood we didn't have better options on short notice — and thought you were dead."
"... Stratt. Why do you have his phone?"
"You know how he is about elevators, and the stress of everyone-... well... It was better for his heart to be induced in the coma now."
"You can't wake him up? Tell him I'm alive?!"
"It would be hard on his health to go in and out so much in a short period. This doesn't change the fact he's the only one that can do this — but I'm sure he'll be very relieved to wake up with you by his side."
"I'll send someone to pick you up."
"...Where am I meeting them?"
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