Eevee

She is technically the first person Senku revived. they went to school together and had many classes. She was incredibly reserved, so their intellectual rivalry was very lowkey. it took senku three years of being in the same class as her to realize she was pretty crap at applying the knowledge she has, (all the tests and classwork is fine because something is directing her to think of the answer.) and that she wasn't some super genius prodigy, her memory was just fucking insane. he immediately starts bugging her about learning new things and shoving resources into her arms at any given moment, plus her own self education of her hobbies and things she likes.

that being said, in terms of knowledge, she knows just as much, if not more, than Senku. but without someone asking questions and pointing her in a direction, she can't really pull that knowledge up herself. the more time she spends with senku, the easier it is for her to pull upon her own knowledge and apply to solve problems. it's still difficult and takes a while.

she was left behind with tsukasa as senku and the besties left for gunpowder. Tsukasa is all like "damn, you must not have been friends, we'll ill make sure you're safe here" and Mizuki sits there in her head like. 'noo sir. I just have the grace of a cat with cerebellar hypoplasia and I would slow them down, so I stayed behind on purpose.' and the goes like, "yeahhh , btw i know a whole lot of ancient traditional apothecary stuff. im assuming you dont want to revive people only for them to get sick and die?" and tsukasas all like "shit, yeah. good thinking."

and then he revived gen and gens all like "oooo doctor doctor! I have to leave soon but I have a *hunch* 👀" and Mizuki is all like, "good luck, dont die!" and hands something off to him in a bag of herbs, dried fruits and nuts. he comes back all beaten n shit and she goes with them on hyogas orders to patch them up so they can stay there longer. she doesn't stay with them. she and gen defect.

after the kingdom dogfight, senku made Mizuki a pen and books and Mizuki starts taking hella notes for him while also continuing her apothecary crap. she has a spare sketchbook that she draws in.

senku and her are such old married couple that no one asks if they're dating and think immediately like yeah senku divorcing Ruri immediately makes sense now. and that leads to like, ryusui being like, "i want in on this relationship" and gen goes "ummmm no I got dibs AND im working on it. get in line" and senku and Mizuki are like "what relationship??" and everyone just looks at them because a day in their life looks like-

wake up in the same bed (cause co sleeping has a whole shit load of benefits, and that makes it logical)

morning routine together (It's cause Mizuki is a clutz, she'll trip and die and that would just be embarrassing)

breakfast together (senku gets distracted by all his science, so he needs to have someone practically spoon feeding him)

separate for a bit to do things senku goes and delegates jobs/ science production and Mizuki collects and preps herbal medicines in a shop that was built for her. (see, they aren't *always* together)

eats lunch together (Mizuki needs to know what's happening to write it down!! plus now is the time to have people come up with concerns n such)

after lunch nap (it boosts productivity in the afternoon! its logical!)

resource check together (2 set of eyes catch more than one)

afternoon science together (Mizuki is a good lab partner they just gotta keep her away from chemicals also the amount of things she knows means the other lab assistants can ask her questions if senku is focused too hard.)

dinner together (everyone eats dinner together... !!! they dont always sit next to eachother !!!)

end of day recap for the books, self-study, and night routine together (its literally just them writing notes and to-do lists while getting ready for bed...)

going to bed together. at the same time. (.........they dont have to explain anything to anyone)

they're not overly pda type stuff. They bicker and fight a lot and poke at each other, and that's their pda. gen and ryusui get that, and when they jump in on it, it just turns into polyandry.