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I lied awake for 3 hrs again this morning and rethought the backstory of how Red and Green met in monsterboy AU. Originally Green was already a married adult with money to spare so picking up a majuu was a hobby with a side of 'my relative knows this stuff so I can probably do it by association', now it's turned into child Green finding a Red outdoors and deciding he should have a majuu of his own. This majuu might be especially difficult and dangerous? Good, I get the coolest majuu. The Red he finds is unclothed and ice cold and weak, so by luck he wins himself a majuu with just a thermos of tea and a coat, but he doesn't know until weeks later when Red has been fostered for a while at the house across from his. Once Oak feels certain about Red's temperament they let the two go exploring together, and Green uses it to coax Red into practicing magic where adults can't tell them it's off limits. Red's body produces so much mana it's not like they'll be able to tell. Throw in a wild youkai attack to demonstrate the maou ability to compel others to follow their orders and gradually allow Green to know Red's magic and mannerisms more intimately than anyone else. Bar mum I guess.
Overall putting them together earlier for longer makes more sense, both to how they understand eachother, and why Green keeps ownership of something so rare. It also dodges the jealousy question with Blue if Red sees Green fall for her and treats it as 'you like her, we get you a her, happy Green.' And a young Blue getting riled up in confusion by them and trying to decipher Red's cryptic ass gesturing in his attempts to sell Team Green. No wonder she grabs him by the ear.
I figured Mr Mime would be replaced by an esper employee at the lab (makes it easier to gauge subjects and knock them out), but haven't settled on what Blue is using. Maybe she's pinched some magic tools and has taught herself some tricks. Green would be interested in that since he's only used magic through Red.