A person can both be alive and have a point in their past where they died. The clinical definition of death is for the heart to stop pumping, which while extremely serious and carrying a 100% death rate if left untreated, can be reversed if the victim is given immediate attention to restore the flow. This has a higher chance of failure, but there have been cases of success.Based off what Kaori has said in Hexadecimal:I can definitely see her simply being a corpse powered by "Cute Girl Magic" or something.You don't have permission to view the spoiler content. Log in or register now.
Then again, I haven't crossed out that mostly everyone are actually just corpses puppeteered by some insane entity. Egg Time:You don't have permission to view the spoiler content. Log in or register now.
In Kaori's case, it could be that her heart ceased beating during a previous incident, quite possibly the same accident that took Sekai and Nozomu from Ami, and that she was saved with immediate response from medical professionals or a bystander that was either trained or walked through what to do in real time. It is also possible that she ended up needing a transplant because of this same accident or for other reasons that lined up perfectly with when the accident happened, allowing Sekai to become a donor of at least some of the necessary parts for this transplant.