Kumiho actually don't become more human if they eat more livers and hearts; they become humanoids if they ABSTAIN from eating hearts and livers. Not many Kumiho do so, but the few that did over the centuries did so out of love, or to protect a beloved human from harm, archieving the secret of becoming human in the process by complete accident.
A type of Nine-Tailed Fox Fey Spirit, common in the Kingdom of Choson. Often mistaken to be their more benign cousin, the Kitsune from the Empire of San, Kumiho are more often than not dangerous, though exeptions, while rare, exist. Female Kumiho, which are a lot more common than males, tend to seduce humans and other humanoids to eat their hearts and livers. This extends the lifespan of Kumiho greatly, and the Kumiho won't have to eat for 100 days. However, some Kumiho believe that eating many human livers and hearts can turn one completely humanoid; or at least makes their disguised forms more humanlike. Some Kumiho specifically seek out this outcome, in order to gain a better life for themselves.