Charles really likes Dr. Ishida. He has a lot of respect for Ryuuken as a professional. And as has been shown, Charles has a thing for the grumpy ones. IT'S JUST SO FUN TO LET THE PRICKLY EXTERIORS ROLL OFF HIS BACK AND ENJOY THE SQUISHINESS THAT HE KNOWS IS INSIDE :x But Charles really does trust Ryuuken a lot as a doctor, and he really helped him when he first arrived in camp, so that was a super-strong first impression. As was Ryuuken's sadness. That also struck a very sensitive place for Charles when he first arrived in camp, given what had just recently happened to him in his life. And Charles is very stubborn about sticking by sad people normally anyways.

Charles is feeling less sad these days and more... tumultuous, so he's probably less with any sympathetic fits of melancholy. But it's still a relief for him to hang out with Ryuuken sometimes and just... not have to be all right or set an example or Be The Adult or save the world.

Reverse, plase?