Eh, saw this on Tumblr, knew no one would ask me so decided to answer them all without being asked, ahahaha.
1. Who cooks?
Oh god, no one. They almost always go out to eat or order something. If they have to eat at the apartment, I guess Sakaki does most of the cooking (I bet Sonoda has tried to teach him some basic survival skills so he doesn't have to worry about finding him dead in his apartment) and Yuki helps. Actually, Yuki is probably at least an average level (home) baker, so maybe they won't die after all!
2. Who's the messiest? The cleanest?
HAHA WELL. Sakaki is a neat freak and a bit of a minimalist. Yuki's a teenage boy. I'm sure he's not a slob, but he's still young and tends to be messy.
3. Who fixes the vehicle after a breakdown?
The auto repair shop. Yuki isn't even old enough to drive, and Sakaki doesn't have time for shit like that.
4. Living space has a leak! Who fixes it?
5. Who buys the groceries?
Probably Yuki, but after the first few times that he came home with way more than they could possibly use in one weekend, I think they started shopping together. (Oh man, there's a fun fic idea...)
6. Going out to eat: Who pays? Who orders the most food? And who has dessert?
AHAHAHA. Sakaki pays, Yuki eats a ton and orders TWO desserts. :p
7. Would they go to the beach?
You know what, I think they would! But they wouldn't spend a lot of time on the actual beach or sunbathing or whatever, I think they would hang around on the boardwalk and shop/eat/play games, and maybe do something like fly a kite, and after that, a nice swim... :3
8. Who knows how to swim? Who doesn't?
Hee, I'm planning a somewhat-AU story where Sakaki's a merman, so he's a really good swimmer. Yuki knows how to swim, but he's just average, and he's not super confident about the ocean... he learned to swim in pool lessons and doesn't have a lot of experience with the actual ocean.
9. Is someone multilingual? Do they try to teach another language to the other? How does it go?
Sakaki's reasonably good at English since his family is involved in international business, but he's probably gotten a little rusty over the last few years because he hasn't needed to use it at all. I'm sure he'll help Yuki with his English homework, though it may devolve into yelling and tears at some point, haha. SAKAKI Y U A TEACHER IF U NOT PATIENT
10. Any pets? Or plants?
Sadly, no, since they can't spend a lot of time at the apartment. Everything would die... maybe they can get an air plant.
11. Baths or showers? Together or separate? Any bubbles or bubble fights?
At school, it's separate baths or showers... or rather, group baths at their respective dorms, individual showers in their rooms. (Hmm, I wonder if Sakaki goes to the group bath, haha...) At the apartment, they'll shower together sometimes, but that usually devolves into shower sex. If the bath's big enough for two, then yesssss shared bath! Yuki would totally bring the bubble bath and use too much, then sculpt it into a funny hat or something on Sakaki's head (like the Arata/Yagami bath scene ahaha).
12. Can they stand silence? Who talks the most? Who talks the least?
Sakaki loves silence, I think he's gone entire weekends at his apartment without speaking, though I'm sure he listens to music (classical). Yuki is okay with silence, but too much and he starts getting uncomfortable and he just can't stop himself from talking. Yuki definitely talks the most, he gets excited so easily and tends to just blurt out whatever he's thinking. (Sakaki finds this very endearing :3)
13. Who stays up late? Who sleeps the most? Does the other have to force them to sleep/wake up?
Sakaki tends to stay up late during the week, catching up on all his various types of work (asst. director stuff and teacher crap) even though he has to get up early, but once the weekend hits GOOD LUCK. Yuki couldn't extract him from that bed even if he had a crowbar. Sakaki sleeps SO MUCH on the weekends, and he has a really hard time getting up early during the week too. He's probably the guy who has three different alarm clocks, including the one that runs away so you can't hit snooze. And he'd die of embarrassment if his coworkers knew it...
14. Who is the highest maintenance? Does the other mind?
Yuki is a bit high maintenance at the beginning of the relationship because he has a hard time believing that someone like Sakaki could truly love him. Sakaki doesn't mind because he's secretly worried about the same thing, he just doesn't show it. They chill out a bit after surviving their first big fight.
15. Vacation ideas: who decides them? Where would they go, if anywhere?
They're doing stuff every weekend, traveling around the country to visit temples, do cultural stuff (museums, galleries, events), check out food festivals, etc etc. They go out most weekends and it all depends on what events are available, but they do decide together on everything. Of course, they both remain flexible and each will do something that's "boring" to them if the other wants to... like that time Yuki made Sakaki take him to Comiket, ahahaha. XD
1. How often do they have sex, if at all?
Every weekend, multiple times if Yuki can get away with being that much of a piglet. :3 Verrrrry rarely during the week if they can't get by with phone sex or just sucking it up and waiting for the next weekend.
2. Who brings ideas? Who initiates?
OH GOD Yuki is always showing up with all sorts of stupid ideas that he saw in like the equivalent of Cosmo, shit like "25 ways to drive your man wild in bed" but only 2 of them would even work because the other 23 require lady equipment or something, but he's determined to try them anyway. Sakaki just tolerates this shit because he figures that's the price you pay for having a younger lover who hasn't figured out exactly what he likes yet, and it's not fair to stifle his creativity, haha. Yuki typically initiates but that's only because he's a horny teenager and he beats Sakaki to it most of the time. XD
3. Any kinks they clash on?
Aww, there was that one time that Yuki got himself some black lace panties (because he heard about a certain other couple's kink from Tomo haha) and wore them allllll day, feeling all sorts of hot and sexy, and when the time finally came for the big reveal, Sakaki was like... NOPE NOPE NOPE. So Yuki was pretty upset about that. Sakaki also won't play at non-con at all, especially while Yuki is a minor.
4. Oddest place they'd have sex?
The backseat of Sakaki's Prius from the future, haha.
5. Favourite positions?
Huh, I haven't thought that far ahead yet. For Yuki, probably all of them, he's just soooo excited about everything.
6. Dom/top? Sub/bottom? Any switches?
Hot teacher on top, underage student on bottom. They do try switching it around sometimes, but Yuki's a terrible top, he just comes within seconds of putting it in. He'll get better with practice. :3
7. Genital headcanons?
UM WHAT. Don't ask me shit like that, haha.
8. Favourite erogenous zones?
Sakaki: PLZ PET MY HAIR. And the rest of me. Light tickling is nice too. (PS nipples)
Yuki: At first, everywhere, he's soooo new and soooo young. :p
9. Quickest turn ons? Immediate turn offs?
Sakaki hates non-con and anything "girly" or whatever, so if Yuki shows up in those lace panties again, there won't be any sex happening, sorry. And Yuki gets all fired up just thinking about Sakaki's hands, he has a MAJOR THING for his hands.
10. First to orgasm? Last to orgasm? Who comes the most? Does someone ever end up unfinished?
Yuki's almost always first, and usually comes more than once. I wouldn't say it happens often, but there are times that Sakaki doesn't finish, and he doesn't really mind. I mean, Yuki can just lend him a hand, so to speak. :p
11. Favourite romantic gestures during sex/orgasm?
I kind of don't even understand this question, haha. Uh, Sakaki likes saying Yuki's name all sexy, does that count? :3
12. How are their afterglows?
Shiny and wonderful? Probably a bit sweaty too, haha.
13. Who's loud? Who's quiet? Does one try to make the other louder/quieter? How?
They're both kind of quiet, Yuki because he's embarrassed and Sakaki because, uh, he's also embarrassed, haha. Sakaki tries to make Yuki make more noise by doing ~things~ to him that he doesn't expect. Well, Yuki's inexperienced so it's not hard to make him squeal a bit. :p But hey, there's something hot about lovers just clinging to each other and panting, trying to hold back and squeaking a bit here and there, right?
14. Lights on or off? Do they look at each other? Or is someone embarrassed?
Lights on, but dimmed, so you get those sexy light/shadow contrasts. They're always looking at each other, but Sakaki's actually the one who gets embarrassed more easily because Yuki is VERY INTENSE. Kind of a big plot point during their very first time. :3
15. Open or closed relationship? Do they sometimes share?
Closed. Yuki's demisexual and Sakaki's not interested in sharing.