It probably like a thing that occurs with how Aven and Ratio canonly meet and start to work together. Aven gets assigned to work with the Guild rep Ratio, bringing along his very curious Stellaron-hosting friend for the ride. (Since they never separate for missions now that he has him privately on his employee list.) Once Ratio realizes that THIS is the Stellaron host he’s heard about, he naturally becomes curious. He sits in on a few experiments on him and is appalled by how the IPC treats him as purely subhuman and animal-like. His respect for how Aven seems to protect him and treat him as an equal grows for this, and he tries his best to be kind to the other during their work time together to show he is not like those researchers that put results above human morale and empathy. (He’s blunt bc it’s Ratio. But I can see him speaking only politely to him and maybe even getting him a snack or something during their mission he saw him eyeing. Consider it a peace offering.)

Aven and Caelus are both hesitant to trust someone of course, but with time he sees how Cae warms up to Ratio and can tell he’s being honest in his politeness and good intentions and has been actively trying to campaign for better treatment of Caelus to the the researchers. Aven had been trying but his influence in the science part of the IPC was minimal. After the hurtful tests suddenly stop and turn much more normal, they know right away it was due to Veritas fighting for it. Caelus gives him a huge hug the next time they see each other for a mission and almost knocks him over, meanwhile Aventurine is grinning ear to ear. “Happy to see you again, doc.”