A = Affection (How affectionate are they? How do they show it?)What do you think? Mirio is the most affectionate boyfriend in the world! He doesn't shy away from PDA. He just loves letting the world know that he's the lucky guy who you chose to be with. Be it hugs, kisses, hand holding, whatever.
B = Best Friend (What are they like as a best friend? How does the friendship start?)
Mirio is a very supportive friend. He will encourage you when you feel down, allow you to vent to him and be there for you to cry on your shoulder. The friendship would most likely start similarly to his friendship with Tamaki. He would notice you were alone, would walk over and strike up a conversation.
C = Comfort (How do they help their partner when they're down? How do they help with a panic attack?)
Cuddles. This boy is a big cuddle monster and that only get's worse when you're down. He will simply wrap his arms around you and stroke your hair. The same when you're having a panic attack.
D = Domestic (Do they want to settle down? How are they with cooking and cleaning?)
Of course Mirio wants to settle down! He wants nothing more. He's not that good at cooking, but he'll do his best, God damn it. The same with cooking! He always tries his best! If it isn't up to your standards, he'll simply try again and again until it is.
E = Ending (If they had to break up with their partner, how would they do it?)
Nah. Not happening. Sorry, bucko.
F = Fiancé (How do they feel about commitment? How quickly do they want to get married?)
Mirio is perfectly fine with commitment. He's completely committed to you! He wants to get married as soon as possible, as soon as you two can afford it!
G = Gentle (How gentle are they, physically and emotionally?)
This is a gentle guy we're talking about here! No matter what, he'll be as gentle as possible, even if you don't want him to be wink wonk. When cuddling, he'll make sure not to hold you too tight since he doesn't really know his own strength.
H = Hugs (Do they like hugs? How often? What are their hugs like?)
His hugs are amazing! Like I said above, they're always gentle because he's honestly terrified of crushing you.