"why did you pick your major? please refrain from using this time to call me a ." 

soohyun, in deadpan, questions, "call the idiot an idiot? now who would do that? i picked my major because it was easy." (in the background, there's a choked "easy?! do you know-")
 

"what was the last thing that you lost? get all metaphysical or something, i eat that up." 

"literally, someone borrowed my handkerchief and hasn't returned it," she answers, shrugging. (said handkerchief, they all know, is on the cabinet, two feet away from where it's supposed to be.) "metaphysically?" she sighs. "apparently my common sense if i'm living with you all and not regretting it."
 

"what do you think our biggest obstacle is in our plan? i mean, apart from us being awful, how do you think we can improve?" 

"besides quitting this plan now or replacing our entire team?" soohyun questions, tucking a loose strand of hair behind her ear. "uncertain. i don't have experience in improving given my currently abysmal level. i only do semi-decently as is with daehyun's support. perhaps ask daehyun how he improved?" 

"how much class are you willing to blow off in order to make us millions?" 

"i already don't go to class except for test and quiz days," soohyun reminds. "the better question would be if you should. my grades don't suffer from missing class. how are yours?"

"finally - what would you answer to claims that you were the one that drank my beer and ate my leftovers last saturday? this is crucial." 

soohyun sighs for the umpteenth time. she holds up a finger. "one. i hate beer; vodka girl, remember?" another finger. "two. it's been a week." another. "three. you ate them the night after while you were drunk again. now let me read in peace." 

starting relationship. 
the two join the club at around the same time. they aren't particularly close or anything initially, but they discover they share one of their major engineering classes together and pair up. studying together and spending a lot of time with one another, especially with the club, they fall together naturally. 
interactions. 
they aren't overtly romantic but just tend to be together, often seen together more than not. despite living in another place, it isn't a surprise to soohyun or her housemates when daehyun spends nights over in their home. (eventually but early in the year, he moves in with his roommate because cheaper rent.) 
daehyun is pretty obvious about his feelings to soohyun who remains oblivious to it all - including her own feelings. her housemates laugh because she is a genius who is essentially doing a biology, chemistry, and engineering major combined into one but doesn't even realize she likes this boy. sheesh. 
soohyun makes a lot of excuses about her actions as "best opposite-gender friends" and doesn't seem to understand no matter what. daehyun, on the other hand, essentially stakes his claim against other males and fends off confessions with the honest "i have someone i like" lines. it's very entertaining. 
extra
they cuddle "platonically" while watching movies. 
soohyun secretly enjoys daehyun calling her "noona." 
daehyun has fought off girls with crushes on soohyun before.
the sweatshirt stealing began when one of his sweatshirts ended up in her laundry by accident; he never got it back. 
daehyun hates getting up in the mornings and often has a "face of death" (in which whoever runs into him will die) but soohyun always has a cup of coffee waiting for him now.
for a good month, their housemates thought their "study sessions" (when soohyun went to daehyun's place) were when they were getting together; they were just studying. literally. 
someone (coughkyungri) got them drunk eventually. soohyun, as it turned out, just got super sleepy and force-cuddled / latched onto the people closest to her. daehyun didn't seem to get drunk but couldn't remember anything at all the next day. 
soohyun thinks daehyun is touchy-feely because he is with her - he even gives her forehad kisses - but she seems to not realize that he is that way only with her. he lets people eyeing soohyun see his affection with a dangerous warning glare.
whenever he's stressed and they hug, daehyun has this thing where he puts his cheek on her hair, breathes deeply, and relaxes. it's adorable to see daehyun so comfortable with her and a girl-crush moment to see her hold him up so reliably. 
their housemates like to tease them, but mostly daehyun because soohyun is too oblivious. one way that the girls like to is pointing out new things about soohyun and watching him blush about it later. (Ex: soohyun got a new perfume - the girls tell him - later, she gives him a hug and bam, red face because ohmygoshshesmellslikevanillanow.)
progression. author's choice