I've never done this oh man can i ask for DMxMM headcanons!?
Mastermind formed a relationship with Demonio a long while back, when Demonio lacked his white, long hair and tired eyes. They’ve bonded over cookies, and later grew closer, with Demonio treating Add with cookies and Add secretly treating Ciel back with awkward hugs and kisses.
As time went on however, Demonio slowly stopped giving Add affection, instead returning them to Diabla instead. Add tries not to show it, but to say he feels hurt is an understatement. The two slowly stop talking for a long time.
They open up again when Add attempts to communicate with Demonio again. Its an awkward conversation, with Add trying to pry Ciel to talk, and him merely responding in one liners. Mastermind… isn’t sure what to say, quickly running out of conversation topics.
Ciel, though having almost stopped completely in verbalizing his affection, ends up trying to pick up some slack in letting Add know he still loves him. The half demon’s hands reach out to stroke through his hair before he left with Diabla. Add wishes Demonio could express it more often, but he appreciates that he gave him something to let him know.
Mastermind tightly hugs the demon one time, glad he came back to visit him. Demonio ends up hugging back. They both held that position for as long as they could.