I headcanon that habit has a serious case of cute aggression. Just an idea im putting out there uwu
(In my writing Flower Kid is of course you or who you put into the universe etc. etc. You get the idea)
Op you are entirely too true. While he will not go absolutely berserk, if you do something what he sees as well bad....expect to have a more "serious" Boris. Almost like a parent. Especially if concerning your health. Being neglectful is not something you do around this man. His aggression I see as him tormenting you basically till you give in. If you feel bad or under the weather it wont take long for you to let him care for you.
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💭: Okay you have a small headache. What you thought was a headache that is. It's not something too much to think about till you get questioned by Boris. You shrug it off as just some stress from your everyday life, work(possibly),school, or just things on your mind. He cocks an eyebrow and leaves you to be while you relax on his and Kamal's recliner in their living room telling you there's some Ibuprofen in the cabinets if needed.
💭: Suspicion rises when you start feeling worse than before even with the medicine in your system. A cough and sniffle from you brings Boris out of his room and standing at where their hallway begins. He walks out, arms crossed with a concerned expression. You know you aren't feeling well but, you can't possibly bother him with something you can take care of right? "Y/N you need to tell me if you are feeling bad. It's no problem at all if I need to take care of you oki?" Boris explains to you. That being said you just can't let him do it.
You wave him off. "I'll be fine, please don't worry. It's just allergies and stuff. It happens this time of year." Although you were completely lying to him which you did /not/ do. The taller man frowned and walked closer to you this time where he's in touching distance. "I do not think these are allergies." Boris starts tapping his foot looking down at you. The averting of eye contact let's him know something is up with you. A turn of the heel he goes to the bathroom rummaging around coming out surprisingly fast dressed in a lab coat he kept after the Habitat while holding a medical bag.
You begin to argue with him while he's standing there with a calm smile placed on his face wagging his finger. "You cannot hide the fact you don't feel well. Now let's go to the kitchen nd' I can take a better look at you there." You protest and tell him "no", this only makes the smile twitch and a growing aggravation come from him. Boris simply nods and at an instant before you can reply back the man has you in his arms taking you to the guest room you stay at while visiting and places you in bed.
"I care about you Y/N and I cannot have you just staying sick and not be healthy again. Now say 'ah." Boris had a spoonful of medicine just for you ready and up to your mouth. You close your mouth tight and shake your head sinking into the pillow. Boris tilts his head at this. " Oh, so you wish not to take the medicine? That is such a shame. I cannot have you though /not/ take it. Ah, well I guess it's time to get more assertive with you,though I don't wish to do that seeing the state you're in...you leave me no choice." It was almost like you knew what was coming and hid under the covers. This did not stop him though.
Boris taking free hand poked at your side making you jump up. "Now,now are you sure we need to bring in the other method to make you take it?" You pop your head from the covers and furrow your brow. "I'm okay Boris really I just-" with all this going on at the last moment you went to open your mouth he popped the spoon right in patting your head. "Now let the medicine go down and rest. I'm not letting you leave especially in the condition your in. I can feel you're warm." You huff and still shake your head leaning back into your pillow.
Seeing you upset with him made the man's smile disappear and he gently patted the covers. "I just want you better Y/N. Same with Kamal if he's feeling bad. I promise to make your stay here enjoyable if you'll just let me help." Still a little mad about it you think about it for a bit and have to agree maybe it /is/ best you let him take care of you while you're ill.
After a moment passed you looked up at him and smile softly nodding. This made him smile back with a much bigger one. "That is all I need to see. You lay back and get rest. You need it." Taking a bigger blanket at the end of the bed he covered you in it as well tucking you in. Your eyes start to narrow down as you snuggle into the warm cozy bed. Slowly after you're asleep Boris takes a seat at the desk in the spare room and leans back getting comfortable himself also going asleep.
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;; I hope this is kinda what you meant by cute aggression. I just kinda took it as him basically pestering till he got you to do what he wanted. I've also been feeling under the weather myself so I wanted to do something soft. Bear with me I haven't written anything since my Hetalia days haha.