Alya starts to watch him very, very closely.
One day she gives him the "Best-Friend talk".
"I saw the way you're looking at her, and as her best friend, it is my duty to inform you: If you ever…."
Marinette saves him, but the damage was done.
He treats her like some fragile piece of art for almost the rest of the day before she tells him to cut it out; she's not a princess.
""
Of course, he can't resist calling her that (it fits), and now they have pet names.
Well, he has a pet name.
For her.
Anyways.