(thank you very much anon#2!! that’s really sweet to hear, i’m ecstatic you like them so much!!)
I grouped the two asks because they were the same.
Fukutomi would be the type to do the small, yet very adoring and sweet, things for his S/O. He wouldn’t want to do something that everyone would see, and eventually mock, but he didn’t want to just do nothing. So, he’d always brush his hand against theirs when everyone was preoccupied with their own business, or he’d give his S/O a light pat on the back, or he’d squeeze their shoulder reassuringly; he would want them to know he’s someone they should depend and trust to be serious about the relationship, even if it’s a bit of a professional one.
Shinkai would definitely always give his S/O snacks and other little gifts throughout the week, as he was all for seeing them smile their cute grin; he loved watching their eyes shine in excitement from something he was the cause of, so he thinks the best way to always get a reaction is with gifts.
Arakita isn’t a very affectionate guy in the first place, so he would rarely ever show affection in a sense to prove his love for his S/O. When he does, though, he tends to rest his forehead on theirs and sit in silence, staring lovingly into their eyes, just listening to their every breath drawn. When he felt he shouldn’t bother them anymore, he’d close his eyes and pull them into a hug, mumbling onto their forehead with a small trace of a smile on his lips.
Toudou would do everything under the sun to show his S/O he loves them. He would send good morning texts, check up on them throughout the day, send good night texts, kiss their cheek, give them quick hugs if they pass each other, complimenting them; everything you can think of. He would want his S/O to know he’s serious about being with them, so he’d show his love in any way possible.
Izumida would be the type to send encouraging texts to his S/O, as he wanted to always check up on them and send them support when he couldn’t be around them. He would find it the easiest way to show affection without doing PDA, as he didn’t want to make his S/O uncomfortable or uneasy around him.
Ashikiba didn’t know any other ways to make his S/O know that he loved them than using physical affection itself, which he was fond of doing, anyways. He loved holding them in his arms; feeling their warmth on his chest would be pretty reassuring for him. When he was questioned on it, he stated that he wanted to make sure they knew he was their getaway when they wanted to block out the rest of the world; his arms were their sanctuary.
Even though he isn’t as smooth and suave as he thinks he is, Kuroda would try to compliment his S/O other when he thought was appropriate and sometimes spontaneously. As he tries to speak his mind, he quickly loses confidence and ends up stuttering out his compliment instead; his S/O wouldn’t mind it, though, as they knew he cared enough to try, and that was enough for them. (One time when he tried, he ended up using the wrong word and changed the meaning of his compliment completely.)
Manami would be pretty casual with his affection, but he’d like kissing the top of his S/O’s head the most. It proves that he holds them close to his heart, and he cares about them very deeply. Even if his S/O would get embarrassed from it, he would prefer no other way of showing them from-the-heart affection.
Doubashi wouldn’t know how to effectively show affection to his S/O, just because it was such a new concept to him that it became awkward for himself whenever he thought about it. He’d do every form possible, hoping he wouldn’t look too embarrassed when he did. The most common thing he’d do is hold his S/O by their shoulders, looking them straight in the eye and telling them he’s ready to kick anyone’s ass if they tried to hurt them, as a form of reassurance for his S/O.
Yuuto’s way of showing his S/O affection would just simply be him reminding them throughout the day that he loves them. He would love watching them get flustered over the three words, but he always wanted them to know that when they’re feeling down in the dumps or mad at the cruel world. He would also, occasionally, hold them in his arms during lunch or after school, “just ‘cause he wanted to feel their warmth.”