Pitou
_____
They realized they loved you rather than wanting to use you as a temporary test subject after they caught themselves daydreaming about doing more humane things with you that they'd never even dream about with the king. These things were rather mundane too.
Cooking with you, reading in the same area as you, doing something as simple as sharing a seat with you on the comfy couch in the living room, and just enjoying each other presence, no need for words.
They also realize they hold you to a different standard compared to Mereum, they're less accepting of the idea of being happy without you, they need you, crave you. Letting you be happy with someone else isn't an option.
______
Shaiapouf
_____
He realized it a little later.
You were in critical condition because of him, and the guilt mixed with grief was too unbearable.
He was confused about why he felt this way all of a sudden, and he had to do some research to come to the realization that he actually did care about you. Maybe not enough to go back to his king's side, but he did shed real tears for you.
_____
Youpi
____
It was after he gained some form of maturity, during the chimera ant arc he hid you far away from everyone and the danger in your lab.
His goal after saving Mereum if he remembered anything about Komugi was to come back and tell you how he felt.
Things changed when he found Mereum dead, of course, he had a newfound hatred for humans but those feelings still lingered in a way he couldn't ignore.
When Mereum came back and gave them the rules, he was slightly reluctant but still had those feelings for you so he didn't throw a fit, instead taking it as an opportunity to see just how deep they go.
___
Cheetu
___
The minute he remembered you as his old S/O from when he was still a human!