Brainiac 5 wants to reassure her again that this is pretty much his bread and butter, but he's so overtaken by emotion that all he can do is open his mouth, and then snaps it shut, unsure of what to say, but then he settles for a soft: "Ah."

Not the most tactful, but it's neither positive or negative-sounding.

But if she's unclear at all of how he's feeling about it, the fact that he tucks it into one of the pockets of his plantsuit--which he then seals, making it so it will be even more difficult to lose--says something.

He snaps his fingers at Koko to get his attention.

"If you wouldn't mind horribly, I could use someone to care for Koko. He's also occasionally useful. You might perhaps have need for his assistance."