Who said “I love you” first
Zuko, very sure of his own feelings, even if he was not sure of Sokka or indeed anything else. Sokka was surprised - not at the depth of Zuko’s feeling for him, but that he said it and the way he did.
Who would have the other’s picture as their phone background
If you think Sokka’s phone background isn’t a goofy photo of the entire GAang together I just don’t know what to tell you. (His phone is full of selcas, many of them featuring his friends, sister, or boyfriend, with varyingly long-suffering or playful reactions. It’s not his background, but Sokka’s favourite photo of Zuko is the one he took of them where Zuko is grinning and kissing his cheek.)
Zuko’s background is a photo of the two of them together, standing side-by-side as they were getting ready to go out with their friends. Neither of them is quite looking at the camera - that was the next photo Katara took - but instead at each other, and they look so warm and fond of each other.
Who leaves notes written in fog on the bathroom mirror
Sokka does. Zuko thinks it’s ridiculous but it still makes him smile fondly.
Who buys the other cheesy gifts
They both do. Sokka is more prone to impulse purchases and Zuko often has no idea what to do with the presents he brings back, or even how to really react other than thanks (which only makes Sokka more determined to bring him more, as it reminds him Zuko isn’t used to anyone trying to spoil him). Zuko often puts way too much thought into the presents he finds for Sokka, and then is tentatively anxious about giving them to him, although he’s always happy with anything Zuko gives him and thinks it’s sweet how romantic Zuko is by nature.
Who initiated the first kiss
Sokka swears it was him who kissed first; Zuko maintains he initiated their first kiss. It was a bit of a blur in the aftermath of a battle and neither of them are likely to actually give way on the matter.
(It was actually Zuko who started it, but Sokka nearly bowled him over responding as soon as the split-second of shock wore off.)
Who kisses the other awake in the morning
Zuko. He used to try not to wake Sokka when he got up in the mornings, but eventually realised that while Sokka loves to sleep in, he also falls back to sleep with enviable ease and he’s warm and pliant and cosy first thing in the morning before he’s properly awake.
Zuko wakes with the sun, spends a few minutes lazing close to Sokka and trading slow, sweet kisses, then gets up to go outside for his morning shifǎ while Sokka curls up and goes back to sleep for a while longer before breakfast.
Who starts tickle fights
Sokka, in everlasting hopes that eventually he will win one. Zuko isn’t very ticklish in most places, however, and he is very good at mastering his reactions, so tickle fights tend not to end well for Sokka, who cannot repress his flailing whatsoever when he’s being tickled.
Who asks who if they can join the other in the shower
Depends on who is heading into bathing rooms or the water first. And it’s often not so much asking as just quietly slipping in after him. They both enjoy the closeness and bathing each other, although sometimes Sokka regrets sliding into Zuko’s bath without warning because his heat tolerance is much higher and sometimes Sokka swears the bathwater could be used to cook with.
Who surprises the other in the middle of the day at work with lunch
Zuko does it more often, but Sokka does from time to time as well, especially when politics are being particularly rough and Zuko is getting frustrated and anxious trying to manage everything.
Who was nervous and shy on the first date
Zuko, who has no faith in his ability to get through a date well. Sokka is working on that with him, but really what helps the most are Sokka’s own awkward flaily moments. (Though his affectionate praise and romantic moments are certainly reassuring.)
Who kills/takes out the spiders
Zuko pretty much ignores them himself but sometimes they freak Sokka out still - he never got used to them growing up on the ice, unlike Zuko who spent his childhood in a biome where insects and other pests thrive. While it’s amusing to watch Sokka startle and brandish a weapon at a pest smaller than his thumbnail, Zuko tends to intervene before he can hurt himself or, more likely, break something in the process of smashing a spider with his boomerang.
The first time Zuko flicked a puff of fire at one to kill it (an actually venomous specimen, on a trip back to Ember Island) Sokka actually shrieked, which startled Zuko in turn because what? what’s wrong? - you threw fire under my nose! - I wouldn’t have hit you! . . . Sokka’s gotten more used to it now, but Zuko still tries to remember to warn him before firebending too close to him, especially indoors.
Who loudly proclaims their love when they’re drunk
Sokka. 100% Sokka. Declarations of love, poetry, compliments, flirting, innuendo, all completely shameless and sometimes very loud. Zuko is still often a little bit surprised, and always gets rather blushy over it.