I found a super cute blog and thought you guys might enjoy it.
Basically the idea is "who would do x, and who would do y in this relationship?" Some of these work really well for some of Arina's couples; others might not. But feel free to browse through that blog and come up with your own. I'm gonna mention a few I really like with some of my fav Arina OTPs and elaborate on a few.
Who gets sick often, who smothers them with affection, tending to their every need to the extent that they too get sick.
Mitsuki gets sick often (probably more susceptible to colds after the tumor's removal); Takuto is her personal servant during these times. Anything she needs? He's got her covered. And yeah, he'd catch her colds. But he'd tell her not to worry and that he'd be fine. But as "fine" as he seems, he's actually worried sick. Probably wondering if the cancer came back, and how on earth would he live without her if that's the case, etc, etc... Wakaouji-sensei's probably on speed dial.
Who has a habit of “borrowing” their significant others clothes, who doesn’t realise until they open the others closet and immediately recognise half of it.
This is a headcanon I have for Maron and Chiaki, actually. Maron steals shirts out of Chiaki's closet on a regular basis, and he never notices until it's laundry day or his closet's empty.
Every day your OTP mutter “can you believe I married you?”; who says it in wonder, who says it in response to their partner doing something ridiculous.
Both Minazuki-kun and Miyako go through this. Every. Single. Day. And it's not always a response to the other being ridiculous. Maybe it's more of a (somewhat sarcastic?) "why did we think this was a good idea?" Then Shinji does something stupid, and they just look at one another and remember they can't leave their son alone for more than 5 minutes.
Bonus points if this happens and Shinji's 20 years old.
When your otp get into a huge argument, who yells “Just take whatever’s yours and leave!”, who picks up the former and walks out the door.
Alternatively: Takuto yells that at Izumi, who just blankly stares at him, says "Okay then" and picks up Meroko and leaves. Boom. Done. No drama, except for Meroko flipping out at her ex.