Awe thank you!
When they are still young and/or getting together, probably insanely jealous but maybe not even realizing it. Touching, marking- anything to showcase. Standing a liiiiiiiiiiittle too closeeee.... everyone fucking knows but they are doing this peacocking mating dance around each other and never admitting it.

Once they're post fight and established? Kaeya probably finds situations like Donna amusing in a weird twisted way. She can dream forever but she will never have a drop. It's his little thing~ so he can smile in knowing and take pride in it. Instead of getting jealous, he gets cocky. Like haha, bitches that's /mine/ and you have no chance. Have a lovely day, Ma'am, (I'm going to go home fuck the man you can only dream about until one of us cries).

I don't honestly think Diluc would mind Kaeya being friendly to get jealous- that's how he is, and he would trust him. Kinda the journey of their coupling to trust each other again- yeh? However, if someone tries to FORCE Kaeya, or makes him uncomfortable, the full brunt of hell is unleashed on the poor soul. It would never be Kaeya's fault.

"You didn't have to throw it /that/ hard, Master Diluc."
"I did."