How Misha takes Shizune and Hisao getting together would depend on her mindset, which in turn would be affected by her disability (If, as pondered, she even has one).
Say, for instance, that Misha has Hemophilia. She joins the student counsel thingie, filled with work that can cause what would be to others trivial scratchs and cuts (Sawing, cutting, so forth) but somewhat dangerous to her. You could view this as a sign of incredible resolve to not let her condition get the best of her, but this is a teenager: It is difficult for people that age to come to those conclusions. Rather, it is a sign of denial, and a dangerous form of denial at that: Misha is attempting to convince herself that her disease is non-existent, that activities that she once enjoyed or learned to enjoy are still within her grasp, that she can continue in a carefree (Well, you know what I mean) manner towards life through an immature wish.
This implies that her incredible cheerfulness is a facade, and may actually spawn a Co-dependency issue that hinges on Shizune. When Shizune begin to drift out to spend time with a boyfriend, you already have an irrational girl becoming jealous, but also a possible scenario that leads to the accidental death of one or more parties (Misha 'jokes' around with a knife, accidentally falls on it, resulting shock triggers a fatal cardiac arrest, the possibilities!).
... Or she could just be a cheerful, charming girl who takes things in stride. No problem there.
[19:33] <!Suriko> i think i could go my whole life without putting my mouth on someone's ass voluntarily
[12:46] == climatic was kicked from #katawa-shoujo by climatic [climatic]
[12:46] <+ContinualNaba> wut
[12:46] <%VCR> wut
[12:47] <+Juno> Qué