"I don't know what kind of being it was," Marco replied, going over his memories very carefully. "It could switch forms pretty much whenever it wanted. Think much more Mystique than what we do. It was very...godlike," he sighed, the word godlike coming out hesitantly, like pulling a tooth. "How Ax got assimilated, I don't know. It all happened before we even found out about it. We don't even know for sure what the thing is. Our confrontation lasted minutes. It wasn't like the Borg, not some technological thing. It just...was."
And then he looked away. He'd been hoping Cassie would not notice the fact that he didn't add her to the list of people on the ship. "You were getting married...to a good guy named Ronnie. I introduced you two, actually. You were in the president's sub-cabinet, advisor for the Hork-Bajir and other resident aliens. You were happier down here. And realistically..." Marco sighed. "And realistically, if we died, we needed an Animorph on the surface who could pick up where we left off." Marco was pretty sure the only reason Jake even told her about it all was because he knew that Cassie would know where Tobias was. He didn't tell her that, of course. "Considering we died, I'd say that probably wasn't a bad idea."
He wasn't lying to her. He could have, it wouldn't even have really weighed on him...but he knew that Jake would never tell her any of this, if he showed up. And she deserved to know.