If you're still doing the ship thing can you do stisaac?
Oh... well since you asked nicely and seem cool, I'll give it my best shot. It's not a pairing I ever considered. (Mind you, I'm a little rusty on Isaac so it might not be 'fandom' accurate.)
Who was the one to propose: If I had to consider it, I would say Stiles, he's the guy with a plan and it be way elaborate and Isaac would just stare at him when he finishes his elaborate proposal plan, roll his eyes, take the ring and don a pair of sunglasses. Like a badass.
Who stressed more over wedding planning: Again Stiles, wanted to make everything perfect. Nearly had a heart attack about it all. Then his best man, Scott (duh), called up Lydia and they sorted through the worst of the wedding planning.
Who decorated the house: Gonna have'ta say Lydia again on this one. She knows her stuff, one of which is decoration. I mean, Stiles tried and Isaac didn't care much in the matter.
Who does the cooking: Isaac he knows what he likes (Mexican food) and he likes to be in control of food.
Who is more organized: Stiles.
Who suggested kids first: It just sorta... came up, really random, they were watching a tv show, don't ask them which they won't say, and mutually agree. Kids... sounds good.
Who’s the cuddler: Isaac, that puppy needs love.
Who’s the big spoon/little spoon: It's not concrete, they like to switch it up, sometimes work will be stressful and one will need more cuddling then the other.
What’s their favorite non-sexual activity: Solvin' mysteries. Idk man.
Who cooks: I really should just not answer this.
Who comes home drunk at 3am: Stiles again, because he can actually get drunk.
Who kills the spiders: Stiles I guess. Mostly because they freak him out but Isaac is kinda like 'take care of it, conquer your fear man.'
Who falls asleep first: They both struggle to sleep, so it varies.
A head canon: (Oh man...err) Sometimes, when Isaac is feeling sad, Stiles will take him to an animal shelter and they'll take care of the animals there both with powers and without them.
Their relationship summed up in a gif:
Do they have any “rituals”? Not that I know of.
Who has the most patience? Isaac, because again, Stiles can't sit still for more than 10 seconds.
