After Ibuki and Mikan start dating Ibuki refers to Mikan as 'My Wife'. Flusters the hell out of her.
Fuyuhiko noticed Peko's like of flowers and so ended up learning a whole bunch of flower language meanings, the problem is he never has a chance to show off this information so he just drops it at random points when they are alone together.
Kyoko teaches Makoto the basics of hand to hand combat, because well she would rather be there it would comfort her a little if her squishy BF got a little less squishy.
When Chihiro is really comfortable around someone or they eat alone, they eat really fast and more than you would expect. The first time Mondo and Taka see this happen they are surprised, before start cheering them on and eating the same way.
Nagito is stupid good at being romantic when he wants to be. He does his best and by god it's better than he thinks it is. it is also something hard to predict because it is random. Hajime has been floored more than once by it and just "What is this for!?" "Your lucky day".
(It leaves you flustered every time but man does it get him a lot of praise and kisses. Which he will take).
Natsumi learned a lot about mechanics from Kazuichi because she genuinely found it interesting as fuck, and also he was just so cute when he was going on about how everything worked.
Also would commission him a lot to make things. He made it... sometimes he asked why and got told 'Don't worry about it" so he decided not to.
Nekomaru was adored by all of Akane's siblings, he might have had to win them over but god damn was he well liked by the family. Even before they really were dating, dating and it honestly was one of the biggest thing that made Akane fall.
Nekomaru also started bringing Akane to sports trainings so she could meet the teams and she ended up getting really good at advising people and gave some really good plays advice because "my gut is telling me something- I think you should listen."
Also man their training sessions went so hard- they would spar from dusk till dawn under the stars and moonlight, laughing and having a good time interspersed between their serious show downs.
Sonia loves Gundham's house- Miss Tanaka adores her, the animals all get along with her and it just a relaxing place to be because no one judges her and no one expects her to be anything but herself.
Also Miss Tanaka is very into true crime in my headcanon because she mayhaps killed Gundham's father shortly after he was born because he was a foul man, and raised Gundham by herself with all the animals on the farm.
But ya I really think Miss. Tanaka and Sonia would get along great.
Sonia and Gundham both have to take trips a lot because Gundham has to go to animal reserves and rescues well Sonia has to travel for diplomatic purposes, so they video call a lot and Sonia hides her phone under her pillow and falls asleep listening to him update his blog.
Also Gundham is very good at stealth and has snuck into Sonia's dorm room to cuddle multiple times, because no foolish mortal rule will keep him from his dark queen. They are both such fucking cuddlers.
I don't think Sakura actually got out that much other than for fighting, but Hina goes all sorts of places and she starts getting Sakura to come along. She ends up having a lot more fun than she would have ever imagine. Like Hina takes her to Karoke and is cheering her on the entire time, it makes her feel great.
Also Hina is constantly on the hunt for cute clothes for her girlfriend so she can express more femininity when she wants too, because you know it has to be a nightmare to find clothes for Sakura she really likes unless they are made custom.
Sakura cooks for Hina all the time. She makes her mochi, ramen all sorts of traditional food. Hina fucking loves it.