Tsumugi has made Shuichi be her living mannequin more than once. Not that Shuichi minds per se, it's nice to see her happy. But he could definitely do without being jabbed in the side with a sewing needle every now and again, though.
Whenever Tsumugi gets on herself about being plain, Shuichi is quick to take her hands in his, kiss her on the cheek, which is very efficient at shutting the easily flustered girl up.
The two came up with pet names for each other, to ease each other's self-hate.
Tsumugi calls Shuichi "Pretty Boy", "Cutie", "Honey", and "Love". Shuichi calls her "Princess", "Angel", "Sweetheart", and "Beautiful". Needless to say, it works pretty well.
Shuichi will happily attend conventions with Shuichi, either to cosplay with her as a couple or simply to cheer her on and keep the creeps away from her. Female cosplayers go through too much shit.
Saihara is also a law enforcement member, so he's always legally allowed to be armed, which comes in handy for scaring off the creeps.
Not that Shuichi would ever kill them for real, of course, although the thought has crossed his mind on more than one occasion.
Tsumugi is the first one to comfort Shuichi after he encounters something traumatic on the job. A dead body, a murder scene, a suicide, a shootout, anything. And she was the one to convince him to start therapy for those traumatic scenes he was forced to witness.
They like to cuddle at night! Namely, Tsumugi burrows herself in Shuichi's chest while Shuichi holds her there. He likes to feel like he's protecting her.
Tsumugi does most of the household chores, while Shuichi simply watches/helps. Tsumugi insists that she wants to do things for him, since only his job makes real money, save for her selling her cosplays. But that's not saying much, considering her limited audience.
Shuichi still wishes Tsumugi would let him do more and share the responsibilities, though.
So occasionally, Tsumugi is awoken to breakfast already being cooked, or she'll be sewing when she smells cleaning supplies, and she knows Shuichi is doing his share of the chores.
Tsumugi likes to do all the cliche date stuff. Y'know, candlelit dinners, cafe dates, coffee shop dates, that sort of thing. She wants their lives to be like a shoujo anime.
Although Shuichi doesn't completely understand that desire of hers, occasionally, he gives in and lets her have things the way she wants.
She's cosplayed for Shuichi for funsies. For example, wearing a revealing maid outfit and some cat ears and flustering her poor timid boyfriend on purpose.
Shuichi gets the first view of her cosplays, and occasionally she lets him make adjustments. For example, changing certain colors he thinks suit her better. But of course, no matter how revealing, Shuichi doesn't say a word about it. Her clothes aren't an invitation to be creepy, after all. So she should be allowed to wear whatever she wants, revealing or otherwise.
He also knows that creeps don't care, they'll hit on her regardless of what she's wearing.
Tsumugi once made a comment that she'd be the perfect housewife, which made Shuichi flush red at the implication. Tsumugi giggled and revealed she was only joking, of course.
Shuichi often buys little jewelry and trinkets he sees while out that reminds him of Tsumugi. Anime posters, pretty sapphire earrings, and many other things.
A very wholesome couple, really! 10/10, very sweet! :D